github: prevent getRepoPermission to raise AttributeError

Sometime the github.repository() method returns None during getRepoPermission,
return 'none' when this happens

Change-Id: If659c28ab2f52b0412bc2dd9e68420169d32ae69
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index 838cba5..1082de0 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -797,6 +797,9 @@
         # Create a repo object
         repository = github.repository(owner, proj)
 
+        if not repository:
+            return 'none'
+
         # Build up a URL
         url = repository._build_url('collaborators', login, 'permission',
                                     base_url=repository._api)