fix: handle deprecated code related to comments app

[phil-davis]

Code paths that caused deprecation warnings in PHP 8 have been corrected.

When running a comments app API acceptance test scenario locally, I got the following output:

$ make test-acceptance-api BEHAT_FEATURE=tests/acceptance/features/apiComments/comments.feature:9

  Scenario: Getting info of comments using files endpoint                                                 # /home/phil/git/owncloud/core/tests/acceptance/features/apiComments/comments.feature:9
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40580 Accepted
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40580 [201]: PUT /remote.php/dav/files/Alice/myFileToComment.txt
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40580 Closing
    Given user "Alice" has uploaded file "filesForUpload/textfile.txt" to "/myFileToComment.txt"          # FeatureContext::userHasUploadedAFileTo()
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40588 Accepted
[Fri Jun 26 21:11:37 2026] strtolower(): Passing null to parameter #1 ($string) of type string is deprecated at /home/phil/git/owncloud/core/apps/dav/lib/Server.php#243
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40588 [207]: PROPFIND /remote.php/dav/files/Alice/myFileToComment.txt
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40588 Closing
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40594 Accepted
[Fri Jun 26 21:11:37 2026] DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated at /home/phil/git/owncloud/core/apps/comments/lib/Dav/EntityCollection.php#174
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40594 [201]: POST /remote.php/dav/comments/files/2147484663
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40594 Closing
    And user "Alice" has commented with content "My first comment" on file "/myFileToComment.txt"         # CommentsContext::userHasCommentedWithContentOnEntry()
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40610 Accepted
[Fri Jun 26 21:11:37 2026] strtolower(): Passing null to parameter #1 ($string) of type string is deprecated at /home/phil/git/owncloud/core/apps/dav/lib/Server.php#243
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40610 [207]: PROPFIND /remote.php/dav/files/Alice/myFileToComment.txt
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40610 Closing
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40612 Accepted
[Fri Jun 26 21:11:37 2026] Undefined array key "{http://owncloud.org/ns}actorDisplayName" at /home/phil/git/owncloud/core/apps/comments/lib/Dav/CommentNode.php#231
[Fri Jun 26 21:11:37 2026] method_exists(): Passing null to parameter #2 ($method) of type string is deprecated at /home/phil/git/owncloud/core/apps/comments/lib/Dav/CommentNode.php#232
[Fri Jun 26 21:11:37 2026] Undefined array key "{http://owncloud.org/ns}isUnread" at /home/phil/git/owncloud/core/apps/comments/lib/Dav/CommentNode.php#231
[Fri Jun 26 21:11:37 2026] method_exists(): Passing null to parameter #2 ($method) of type string is deprecated at /home/phil/git/owncloud/core/apps/comments/lib/Dav/CommentNode.php#232
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40612 [207]: REPORT /remote.php/dav/comments/files/2147484663
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40612 Closing
    And user "Alice" should have the following comments on file "/myFileToComment.txt"                    # CommentsContext::checkComments()
      | user  | comment          |
      | Alice | My first comment |
[Fri Jun 26 21:11:37 2026] 127.0.0.1:40614 Accepted
[Fri Jun 26 21:11:38 2026] 127.0.0.1:40614 [207]: PROPFIND /remote.php/dav/files/Alice/myFileToComment.txt
[Fri Jun 26 21:11:38 2026] 127.0.0.1:40614 Closing
    When user "Alice" gets the following properties of folder "/myFileToComment.txt" using the WebDAV API # WebDavPropertiesContext::userGetsPropertiesOfFolder()
      | propertyName       |
      | oc:comments-href   |
      | oc:comments-count  |
      | oc:comments-unread |
    Then the HTTP status code should be "201"                                                             # FeatureContext::thenTheHTTPStatusCodeShouldBe()
    And the single response should contain a property "oc:comments-count" with value "1"                  # WebDavPropertiesContext::theSingleResponseShouldContainAPropertyWithValue()
    And the single response should contain a property "oc:comments-unread" with value "0"                 # WebDavPropertiesContext::theSingleResponseShouldContainAPropertyWithValue()
    And the single response should contain a property "oc:comments-href" with value "%a_comment_url%"     # WebDavPropertiesContext::theSingleResponseShouldContainAPropertyWithValue()

There are 3 different deprecation warnings emitted. This PR fixes them:

strtolower(): Passing null to parameter #1 ($string) of type string is deprecated at /home/phil/git/owncloud/core/apps/dav/lib/Server.php#243

DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated at /home/phil/git/owncloud/core/apps/comments/lib/Dav/EntityCollection.php#174

method_exists(): Passing null to parameter #2 ($method) of type string is deprecated at /home/phil/git/owncloud/core/apps/comments/lib/Dav/CommentNode.php#232

[DeepDiver1975]

Reviewed as maintainer. LGTM — clean, minimal, behavior-preserving null-safety fixes for the three PHP 8 deprecation warnings surfaced by the apiComments acceptance scenario:

• CommentNode::getProperties() — $this->properties[$property] ?? null plus a null guard avoids method_exists(null, …) when a client requests an unmapped property; unknown props are skipped exactly as before, and this also silences the related 'Undefined array key' notice.
• EntityCollection::setReadMarker() — new \DateTime($value ?? 'now') preserves the prior new \DateTime(null) semantics (null was always interpreted as 'now').
• Server.php PROPFIND handler — strtolower((string) …) maps a missing Depth header (null) to '', which never matched 'infinity' before either, so behavior is unchanged.

All three are equivalent to the pre-PHP-8 behavior with no functional change. Changelog fragment changelog/unreleased/41656 is present and follows TEMPLATE (Bugfix: …, past-tense body, primary PR ID). CI is green including the PHP Unit matrix and the apiComments acceptance suite. Approving.