If there is no ZUUL_REF, don't fetch it.

Periodic jobs don't get a ZUUL_REF, so don't try to check it out,
just fall back to checking otu the branch.

Change-Id: Id2769d70800a03ccf49012c6f3fce14cffec9671

devstack-vm-gate-wrap.sh

@@ -120,7 +120,8 @@ function setup_workspace {
       fi
 
       # See if Zuul prepared a ref for this project
-      if git fetch https://review.openstack.org/p/$PROJECT $ZUUL_REF; then
+      if [ "$ZUUL_REF" != "" ] && \
+          git fetch https://review.openstack.org/p/$PROJECT $ZUUL_REF; then
         # It's there, so check it out.
         git checkout FETCH_HEAD
         git reset --hard FETCH_HEAD