Object to be skipped from the toSearchableArray method

This allows the current object to be skipped, just by returning false, empty array or null in the toSearchableArray method of the model.
Ref to issue: typesense#24

src/Typesense.php

@@ -156,6 +156,7 @@ public function deleteDocuments(Collection $collectionIndex, array $query): arra
      */
     public function importDocuments(Collection $collectionIndex, $documents, string $action = 'upsert'): \Illuminate\Support\Collection
     {
+        $documents = collect($documents)->filter()->toArray();
         $importedDocuments = $collectionIndex->getDocuments()
                                              ->import($documents, ['action' => $action]);