[git-cl-try] Handle absence of builds

If there are no builds in build search response, the response is {}.
Handle this case.

R=qyearsley

Bug: 849361
Change-Id: I9f0472e140698a1d1c5b98b301bc562ac457785b
Reviewed-on: https://chromium-review.googlesource.com/1085899
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>

git_cl.py

@@ -351,7 +351,7 @@ def _buildbucket_retry(operation_name, http, *args, **kwargs):
       raise BuildbucketResponseException(msg)
 
     if response.status == 200:
-      if not content_json:
+      if content_json is None:
         raise BuildbucketResponseException(
             'Buildbucket returns invalid json content: %s.\n'
             'Please file bugs at http://crbug.com, '