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.
Built from https://develop.svn.wordpress.org/trunk@51625


git-svn-id: http://core.svn.wordpress.org/trunk@51231 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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;

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51624';
+$wp_version = '5.9-alpha-51625';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.