my_activity: Don't crash if email unset.

The script was crashing because of a missing 'email' value from some
gerrit results. Harden against the missing value.

BUG=None

Change-Id: I0f267f130e60e56d532ac2ec1a99ab108a3181e9
Reviewed-on: https://chromium-review.googlesource.com/1161504Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

my_activity.py

@@ -400,7 +400,7 @@ class MyActivity(object):
     ret['review_url'] = '%s://%s/%s' % (protocol, url, issue['_number'])
 
     ret['header'] = issue['subject']
-    ret['owner'] = issue['owner']['email']
+    ret['owner'] = issue['owner'].get('email', '')
     ret['author'] = ret['owner']
     ret['created'] = datetime_from_gerrit(issue['created'])
     ret['modified'] = datetime_from_gerrit(issue['updated'])