ci: skip modified target validation when no targets changed

[juliosuas]

Summary

• Add the missing guard to the Validate modified targets step so deletion-only manifest changes do not run the full live target validation suite with an empty --chunked-sites value.
• Keep the change limited to .github/workflows/validate_modified_targets.yml.

Why

The workflow already discovers changed targets and skips manifest validation, summary generation, and commenting when changed_targets is empty. The pytest step was the only step without the same guard, so deletion-only changes produced an empty CHANGED_TARGETS value and accidentally ran validation against the entire target catalog.

Proof

• PATH=.venv/bin:$PATH .venv/bin/python -m pytest -q --tb=short -m 'not online and not validate_targets' -> 14 passed, 975 deselected
• PATH=.venv/bin:$PATH .venv/bin/python -m pytest -q tests/test_manifest.py::test_validate_manifest_against_local_schema tests/test_ux.py tests/test_unicode.py -> 10 passed
• .venv/bin/python -m ruff check -> All checks passed
• git diff --check upstream/master -> passed
• Parsed .github/workflows/validate_modified_targets.yml with PyYAML -> passed
• Local simulation of the deletion-only case confirmed changed_targets is empty for removed targets GNOME VCS,Patched, so the guarded validation step is skipped.