fix(files_external/SMB): Use 'null' explicitly for no workgroup

Robin Appleman's SMB library is designed so that 'null' is to be used
when no explicit workgroup is set:

    final class BasicAuth implements IAuth {
        /** @var string */
        private $username;
        /** @var string|null */
        private $workgroup;
        /** @var string */
        private $password;
    //...

However, it previous was loose with its checks and would still treat
any falsy value (e.g. an empty string) as "not specified" when forming
arguments for the various underlying utilities, such as `smbclient`.

icewind/SMB@4a93467905 updated it's handling to be more strict and as
such will now treat empty strings as distinct from null values for
workgroups. An empty value for workgroup often doesn't make sense,
unless the backend tool happens to convert it to "WORKGROUP". In the
case of the `smbclient` utility, passing an empty string for the
$workgroup paramters results in `-W ''` being used which is invalid
syntax and so the invocation fails resulting in the user seeing an
`[Icewind\SMB\Exception\ConnectionRefusedException]` error. Therefore,
it is important to always pass 'null' to this constructor for $workgroup
when there isn't one.

Most paths in the 'files_external' app already handle this correctly,
but this now altered path originally just took the raw value from the
app backend and passed it as-is. Assuming this user left the "Domain"
box empty in the frontend, this would be an empty string. Now the
the SMB library's interface is correctly honored in that case.

Fixes #58445.

Signed-off-by: Christian Heimlich <chris@pcserenity.com>

Author: oblivioncth

apps/files_external/lib/Lib/Backend/SMB.php

@@ -70,7 +70,7 @@ public function manipulateStorageConfig(StorageConfig &$storage, ?IUser $user =
 
 			$smbAuth = new BasicAuth(
 				$storage->getBackendOption('user'),
-				$storage->getBackendOption('domain'),
+				$storage->getBackendOption('domain') ?: null,
 				$storage->getBackendOption('password')
 			);
 		} else {