Fix null test plan database error

Summary:
Some Differential fields are not nullable; when Test Plan is switched to non-required mode we can end up trying to save a null value to a non-nullable column (see D2193).

(I should probably just alter the schema to make these fields nullable, but that might have farther-reaching effects.)

Test Plan: Reproduced error, applied patch, no more error.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2200

Author: epriestley

src/applications/differential/field/specification/testplan/DifferentialTestPlanFieldSpecification.php

@@ -19,7 +19,7 @@
 final class DifferentialTestPlanFieldSpecification
   extends DifferentialFieldSpecification {
 
-  private $plan;
+  private $plan = '';
 
   // NOTE: This means "uninitialized".
   private $error = false;
@@ -29,7 +29,7 @@ public function shouldAppearOnEdit() {
   }
 
   protected function didSetRevision() {
-    $this->plan = $this->getRevision()->getTestPlan();
+    $this->plan = (string)$this->getRevision()->getTestPlan();
   }
 
   public function setValueFromRequest(AphrontRequest $request) {
@@ -77,7 +77,7 @@ public function getCommitMessageKey() {
   }
 
   public function setValueFromParsedCommitMessage($value) {
-    $this->plan = $value;
+    $this->plan = (string)$value;
     return $this;
   }