Fix handling of implicit comment transaction in paste creation

Summary:
Fixes T9850. The `getComment()` test should be a `hasComment()` test, in order to discard empty comments.

Also backport a couple of future fixes which can get you into trouble if you reconfigure forms in awkward ways.

Test Plan: Created a new paste without a comment.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9850

Differential Revision: https://secure.phabricator.com/D14571

src/applications/transactions/editfield/PhabricatorEditField.php

@@ -320,15 +320,27 @@ public function readValueFromSubmit(AphrontRequest $request) {
   }
 
   protected function getValueExistsInSubmit(AphrontRequest $request, $key) {
-    return $this->getHTTPParameterType()->getExists($request, $key);
+    $type = $this->getHTTPParameterType();
+
+    if ($type) {
+      return $type->getExists($request, $key);
+    }
+
+    return false;
   }
 
   protected function getValueFromSubmit(AphrontRequest $request, $key) {
     return $this->getHTTPParameterType()->getValue($request, $key);
   }
 
   protected function getDefaultValue() {
-    return $this->getHTTPParameterType()->getDefaultValue();
+    $type = $this->getHTTPParameterType();
+
+    if ($type) {
+      return $type->getDefaultValue();
+    }
+
+    return null;
   }
 
   final public function getHTTPParameterType() {

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -1888,7 +1888,7 @@ protected function filterTransactions(
     }
 
     foreach ($no_effect as $key => $xaction) {
-      if ($xaction->getComment()) {
+      if ($xaction->hasComment()) {
         $xaction->setTransactionType($type_comment);
         $xaction->setOldValue(null);
         $xaction->setNewValue(null);