Don't try to add empty labels

[davidsansome]

When AllLabels is the empty string (as is the case in https://code.google.com/p/clementine-player/issues/detail?id=430) github rejects the empty label with this error:

Traceback (most recent call last): File "migrateissues.py", line 396, in process_gcode_issues(existing_issues) File "migrateissues.py", line 293, in process_gcode_issues github_issue = add_issue_to_github(issue) File "migrateissues.py", line 101, in add_issue_to_github github_labels = [github_label(label) for label in issue['labels']] File "migrateissues.py", line 68, in github_label return label_cache.setdefault(name, github_repo.create_label(name, color)) File "/usr/local/lib/python2.7/dist-packages/github/Repository.py", line 882, in create_label input=post_parameters File "/usr/local/lib/python2.7/dist-packages/github/Requester.py", line 168, in requestJsonAndCheck return self.check(*self.requestJson(verb, url, parameters, headers, input, cnx)) File "/usr/local/lib/python2.7/dist-packages/github/Requester.py", line 176, in check raise self.__createException(status, output) github.GithubException.GithubException: 422 {u'documentation_url': u'http://developer.github.com/v3/issues/labels/#create-a-label', u'message': u'Validation Failed', u'errors': [{u'field': u'name', u'code': u'missing_field', u'resource': u'Label'}]}