git-cl issue 0: Don't operate on empty description

TBR=tandrii

Bug: 741648
Change-Id: If9bcab1892e30ea5fae127302da12f0d9a201cb8
Reviewed-on: https://chromium-review.googlesource.com/570181
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -1472,7 +1472,7 @@ class Changelist(object):
             codereview_server)
     else:
       desc = self.GetDescription()
-      if git_footers.get_footer_change_id(desc):
+      if desc and git_footers.get_footer_change_id(desc):
         print('WARNING: The change patched into this branch has a Change-Id. '
               'Removing it.')
         RunGit(['commit', '--amend', '-m',