Fix logic for discovering new branches

When a new branch exists on gerrit, but has never been seen by RTD
we need to trigger a job so that the branch is discovered
we can then mark it active in the following step.

API changed under out feet, where a 404 was returned before
we now get a 200 with the string null.
Change code to explicitly match the returned string of
"null"

Issue-Id: RELENG-3027
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Change-Id: I769b66384f5b53d87527faeb0f467350a847b65c