Setting all empty vals to 'null'.

src/SolrEngine.php

@@ -21,10 +21,10 @@ class SolrEngine extends Engine
             $searchableArray = $model->toSearchableArray();
             foreach ($searchableArray as $key => $value) {
                 if (empty($value)) {
-                    if (is_string($value)) {
-                        $value = 'NOT_SET';
-                    } else if (is_array($value)) {
-                        $value = ['NOT_SET'];
+                    if (is_array($value)) {
+                        $value = ['null'];
+                    } else {
+                        $value = 'null';
                     }
                     $searchableArray[$key] = $value;
                 }