ci: skip PHPCS step when a PR changes no PHP files

The PHPCS inspection builds a list of changed `.php` files and passes
them to `phpcs`. When a PR touches no PHP files (e.g. a readme-only
change), that list is empty and `phpcs` runs with no file arguments,
falling back to the `<file>.</file>` directive in `phpcs.xml` and
scanning the entire repository -- failing on pre-existing violations
unrelated to the PR.

Guard the step with `if: steps.changes.outputs.files != ''` so it is
skipped when there are no PHP files to inspect.

Author: rafsan096

.github/workflows/wpcs.yml

@@ -37,6 +37,7 @@ jobs:
           echo "files=$FILES" >> $GITHUB_OUTPUT
       
       - name: Detect coding standard violations
+        if: steps.changes.outputs.files != ''
         run: |
           phpcs -i
           vendor/bin/phpcs ${{ steps.changes.outputs.files }} -q --report=checkstyle | cs2pr --graceful-warnings