Fix incorrect construction of subtype map when validating "subtype" transactions against non-subtypable objects

Summary:
Fixes T13389. Currently, we try to "newSubtypeMap()" unconditionally, even if the underlying object does not support subtypes.

  - Only try to build a subtype map if subtype transactions are actually being applied.
  - When subtype transactions are applied to a non-subtypable object, fail more explicitly.

Test Plan: Clicked "Make Editable" in a fresh Calendar transaction form, got an editable form instead of a fatal from "newSubtypeMap()". (Calendar events are not currently subtypable.)

Maniphest Tasks: T13389

Differential Revision: https://secure.phabricator.com/D20741

src/applications/transactions/xaction/PhabricatorEditEngineSubtypeTransaction.php

@@ -25,11 +25,30 @@ final class PhabricatorEditEngineSubtypeTransaction
   }
 
   public function validateTransactions($object, array $xactions) {
-    $map = $object->getEngine()
+    $errors = array();
+
+    if (!$xactions) {
+      return $errors;
+    }
+
+    $engine = $object->getEngine();
+
+    if (!$engine->supportsSubtypes()) {
+      foreach ($xactions as $xaction) {
+        $errors[] = $this->newInvalidError(
+          pht(
+            'Edit engine (of class "%s") does not support subtypes, so '.
+            'subtype transactions can not be applied to it.',
+            get_class($engine)),
+          $xaction);
+      }
+      return $errors;
+    }
+
+    $map = $engine
       ->setViewer($this->getActor())
       ->newSubtypeMap();
 
-    $errors = array();
     foreach ($xactions as $xaction) {
       $new = $xaction->getNewValue();