Converting empty values to 'null' before sending them to Solr.

src/SolrEngine.php

@@ -18,7 +18,18 @@ class SolrEngine extends Engine
     {
         $update = \Solr::createUpdate();
         $models->each(function ($model) use (&$update) {
-            $document = $update->createDocument($model->toSearchableArray());
+            $searchableArray = $model->toSearchableArray();
+            foreach ($searchableArray as $key => $value) {
+                if (empty($value)) {
+                    if (is_string($value)) {
+                        $value = 'null';
+                    } else if (is_array($value)) {
+                        $value = ['null'];
+                    }
+                    $searchableArray[$key] = $value;
+                }
+            }
+            $document = $update->createDocument();
             $update->addDocument($document);
         });
         $this->executeStatement($update);