check_source: when kind is None then it can be a new package

if package_kind() returned None then the submission can be a new package
submission, check_source should continue the process instead of
decline the request.

check_source.py

@@ -82,7 +82,7 @@ class CheckSource(ReviewBot.ReviewBot):
         if kind == 'meta':
             self.review_messages['accepted'] = 'Skipping all checks for meta packages'
             return True
-        elif kind != 'source':
+        elif (kind is not None and kind != 'source'):
             self.review_messages['accepted'] = 'May not modify a non-source package of type {}'.format(kind)
             return False