Code Modernization: Check the input type in validate_file().

This fixes a `preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated` notice on PHP 8.1.

The behavior for `null` and `string` input is covered by the existing `Tests_Functions::test_validate_file()` test.

Effect: Errors down by 238, assertions up by 1920, failures down by 1.

Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51625 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5709,6 +5709,10 @@ function iis7_supports_permalinks() {
  * @return int 0 means nothing is wrong, greater than 0 means something was wrong.
  */
 function validate_file( $file, $allowed_files = array() ) {
+	if ( ! is_scalar( $file ) || '' === $file ) {
+		return 0;
+	}
+
 	// `../` on its own is not allowed:
 	if ( '../' === $file ) {
 		return 1;