Fix two bugs with Config's Edit controller.

Summary:
* When we restored to the default value, we did, in fact delete the row from the
  database, but then a few lines later down, we saved it again. This patch causes
  the controller to return early on delete, like it was supposed to do to begin
  with.
* When checking the user's input value for `null` (since PHP's JSON encoder will
  return `null` on failure), check the value that the user gave, not the value
  that we default to (which is often `null` anyway). Oops.

Test Plan:
* Saved an empty text field and saw the delete work properly and NOT get
  re-added.
* Put `null` in the text field, and saved successfully.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4300

src/applications/config/controller/PhabricatorConfigEditController.php

@@ -42,7 +42,7 @@ public function processRequest() {
       $new_value = $request->getStr('value');
       if (strlen($new_value)) {
         $json = json_decode($new_value, true);
-        if ($json === null && strtolower($value) != 'null') {
+        if ($json === null && strtolower($new_value) != 'null') {
           $e_value = 'Invalid';
           $errors[] = 'The given value must be valid JSON. This means, among '.
             'other things, that you must wrap strings in double-quotes.';
@@ -53,6 +53,8 @@ public function processRequest() {
       } else {
         // TODO: When we do Transactions, make this just set isDeleted = 1
         $config_entry->delete();
+        return id(new AphrontRedirectResponse())
+          ->setURI($config_entry->getURI());
       }
 
       $config_entry->setValue($value);