Only validate form subtype edits if subtype transactions are present

Summary: Fixes T12347. Ref T12314. Validation gets called no matter what, but is only relevant if the form supports subtypes.

Test Plan: Marked/unmarked a Paste form as editable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12347, T12314

Differential Revision: https://secure.phabricator.com/D17457

src/applications/transactions/editor/PhabricatorEditEngineConfigurationEditor.php

@@ -57,6 +57,7 @@ final class PhabricatorEditEngineConfigurationEditor
         }
         break;
       case PhabricatorEditEngineConfigurationTransaction::TYPE_SUBTYPE:
+        if ($xactions) {
           $map = $object->getEngine()
             ->setViewer($this->getActor())
             ->newSubtypeMap();
@@ -73,6 +74,7 @@ final class PhabricatorEditEngineConfigurationEditor
               pht('Subtype "%s" is not a valid subtype.', $new),
               $xaction);
           }
+        }
         break;
     }