fix: Use return null as default in validate_file_to_edit()

Replace the default wp_die() with return null to preserve
the existing runtime behavior for validate_file() code 2
(absolute Windows drive paths). This gives westonruter
the option to revert to the wp_die() approach if preferred.

Author: apermo

src/wp-admin/includes/file.php

@@ -734,7 +734,7 @@ function wp_tempnam( $filename = '', $dir = '' ) {
  * @param string   $file          File the user is attempting to edit.
  * @param string[] $allowed_files Optional. Array of allowed files to edit.
  *                                `$file` must match an entry exactly.
- * @return string|never Returns the file name on success, dies on failure.
+ * @return string|null|never Returns the file name on success, null in case of absolute Windows drive paths, and dies on failure.
  */
 function validate_file_to_edit( $file, $allowed_files = array() ) {
 	$code = validate_file( $file, $allowed_files );
@@ -751,8 +751,9 @@ function validate_file_to_edit( $file, $allowed_files = array() ) {
 			// wp_die( __('Sorry, cannot call files with their real path.' ));
 
 		case 3:
-		default:
 			wp_die( __( 'Sorry, that file cannot be edited.' ) );
+		default:
+			return null;
 	}
 }