Fix warning in grabFixture() when _fixtures is empty (#70)

Author: sunnyphp

src/Codeception/Module/Yii2.php

@@ -510,6 +510,10 @@ public function haveFixtures($fixtures)
      */
     public function grabFixtures()
     {
+        if (!$this->loadedFixtures) {
+            return [];
+        }
+
         return call_user_func_array(
             'array_merge',
             array_map( // merge all fixtures from all fixture stores