fix: use strict equality check for return code validation

Changed the condition from `!$ret` to `$ret === 0` to properly validate the return code. This uses strict equality to explicitly check if the return code is 0 (success), making the intent clearer and avoiding potential issues with truthy/falsy value coercion.

Author: jordikroon

configure.php

@@ -1028,7 +1028,7 @@ function xml_validate_jing()
     if ( ! is_array( $out ) )
         $out = [];
 
-    if ( !$ret )
+    if ( $ret === 0 )
     {
         echo "done.\n";
         return;