optional index casting

Author: nekufa

src/Space.php

@@ -87,7 +87,7 @@ public function addProperty(string $name, string $type, array $config = [])
         }
     }
 
-    public function castIndex(array $fields): ?array
+    public function castIndex(array $fields, bool $optionalIndex = false): ?array
     {
         foreach ($this->indexes as $index) {
             if ($index['fields'] == $fields) {
@@ -117,6 +117,10 @@ public function castIndex(array $fields): ?array
             }
         }
 
+        if ($optionalIndex) {
+            return null;
+        }
+
         throw new Exception("Index casting failure");
     }