Allows NULL on relationship with AJAX request (thedevdojo#4249)

When jSelect ask for the possible relationships the response ignore the required option of the field.

This will ad dback the functionality, it also prevents the output of the 'NONE' option when using the search query

src/Http/Controllers/VoyagerBaseController.php

@@ -661,6 +661,14 @@ public function relation(Request $request)
                 }
 
                 $results = [];
+
+                if (!$row->required && !$search) {
+                    $results[] = [
+                        'id'   => '',
+                        'text' => __('voyager::generic.none'),
+                    ];
+                }
+
                 foreach ($relationshipOptions as $relationshipOption) {
                     $results[] = [
                         'id'   => $relationshipOption->{$options->key},