Fix multiple truthy expressions in eval (#164)

I don't know why this wasn't hit on dev or master...but we hit it on prod

.taskcluster.yml

@@ -45,7 +45,7 @@ tasks:
               $switch:
                   'tasks_for[:19] == "github-pull-request"': ${event.pull_request.base.ref}
                   'tasks_for == "github-push" && event.base_ref': ${event.base_ref}
-                  'tasks_for == "github-push"': ${event.ref}
+                  'tasks_for == "github-push" && !(event.base_ref)': ${event.ref}
                   'tasks_for == "github-release"': ''
                   'tasks_for in ["cron", "action"]': '${push.branch}'
                   'tasks_for == "pr-action"': '${push.base_branch}'