MDL-79642 qbank_tagquestion: Fix Return type err

Previously if $CFG->usetags was not enabled the course question bank
page would show:

  Exception - qbank_tagquestion\plugin_feature::get_question_filters():
  Return value must be of type array, none returned

This is now fixed.

question/bank/tagquestion/classes/plugin_feature.php

@@ -41,5 +41,7 @@ class plugin_feature extends plugin_features_base{
                 new tag_condition($qbank),
             ];
         }
+
+        return [];
     }
 }