Using NOT_SET for empty vals.

src/SolrEngine.php

@@ -22,9 +22,9 @@ class SolrEngine extends Engine
             foreach ($searchableArray as $key => $value) {
                 if (empty($value)) {
                     if (is_string($value)) {
-                        $value = 'null';
+                        $value = 'NOT_SET';
                     } else if (is_array($value)) {
-                        $value = ['null'];
+                        $value = ['NOT_SET'];
                     }
                     $searchableArray[$key] = $value;
                 }