Issue #3422669: If new value is of array type, check if old too before compare

Closes #3422669

src/ArrayDiff.php

@@ -84,7 +84,7 @@ class ArrayDiff {
     }, ARRAY_FILTER_USE_BOTH);
 
     array_walk($changed, function (&$changedItem, $key) use ($old) {
-      if (is_array($changedItem) && !is_null($old[$key])) {
+      if (is_array($changedItem) && !is_null($old[$key]) && is_array($old[$key])) {
         $changedItem = $this->diff($old[$key], $changedItem);
       }
       else {