Hande write-root-marker in tag pipeline

The tag-pipeline does not set zuul.branch, so define it to an empty
string if it's unset so that the script passes.

Change-Id: Iddcf722abec5c6a69f40cb109e5563a698ab349c

roles/write-root-marker/tasks/main.yaml

@@ -1,4 +1,4 @@
 - name: Write marker text
   copy:
     dest: "{{ root_marker_dir }}/.root-marker"
-    content: "Project: {{ zuul.project.name }} Branch: {{ zuul.branch }} Build: {{ zuul.build }} Revision: {{ zuul.ref }}"
+    content: "Project: {{ zuul.project.name }} Branch: {{ zuul.branch | default('') }} Build: {{ zuul.build }} Revision: {{ zuul.ref }}"