Fix crash were CL has no reviewers

Currently git cl owners tries to fetch the current reviewers of a CL
from gerrit. However, this fails with a KeyError when there are no
reviewers currently. This cl fixes that bug.

Bug: 895722
Change-Id: I78a998ee234d207523aa2cef4b5058ba7cfecbc2
Reviewed-on: https://chromium-review.googlesource.com/c/1330122
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

git_cl.py

@@ -3156,7 +3156,7 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
 
   def GetReviewers(self):
     details = self._GetChangeDetail(['DETAILED_ACCOUNTS'])
-    return [reviewer['email'] for reviewer in details['reviewers']['REVIEWER']]
+    return [r['email'] for r in details['reviewers'].get('REVIEWER', [])]
 
 
 _CODEREVIEW_IMPLEMENTATIONS = {