git-cl set_close: Don't fail on branches with no issue

R=tandrii@chromium.org

Bug: 258661
Change-Id: Iee910d6d01df4c675523abdaf4e505364f94c4aa
Reviewed-on: https://chromium-review.googlesource.com/651233
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -5692,7 +5692,8 @@ def CMDset_close(parser, args):
   cl = Changelist(auth_config=auth_config, issue=options.issue,
                   codereview=options.forced_codereview)
   # Ensure there actually is an issue to close.
-  cl.GetDescription()
+  if not cl.GetIssue():
+    DieWithError('ERROR No issue to close')
   cl.CloseIssue()
   return 0