Fix job display in web ui

If a job variant has no branch matchers, we now return null rather
than the empty list.  This is more correct since it better reflicts
Zuul's internal state, so let's keep this behavior, but adjust the
web ui to expect a null value.

Change-Id: I0ba109a65b39e06433223351a0040642e415368d

web/src/containers/job/Job.jsx

@@ -40,7 +40,7 @@ class Job extends React.Component {
          for one branch we don't get the branches.  This might be a
          bug.  In this case, use the source context branch (i.e. where
          it's defined */
-      if (variant.branches.length === 0) {
+      if (variant.branches === null || variant.branches.length === 0) {
         title = variant.source_context.branch
       } else {
         variant.branches.forEach((item) => {