Themes: Ensure WP_Theme::get_files() doesn't return unexpected values.

This change filters out empty entries from `WP_Theme::get_files()` before returning the array of files. This avoid returning an entry with `false` value when the directory of the theme does not exist.

Props dd32, opurockey, Rahmohn, audrasjb.
Fixes #53599.

Built from https://develop.svn.wordpress.org/trunk@53253


git-svn-id: http://core.svn.wordpress.org/trunk@52842 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme.php

@@ -1210,7 +1210,7 @@ final class WP_Theme implements ArrayAccess {
 			$files += (array) self::scandir( $this->get_template_directory(), $type, $depth );
 		}
 
-		return $files;
+		return array_filter( $files );
 	}
 
 	/**