Use find instead of ls for determining dir emptiness

ls foo/* isn't a great way to do this - but find, as is usually
the case, solves all problems.

Change-Id: Ic30b36ccee41e9b74e187eb3cdab0359f659a1c9

roles/merge-output-to-logs/tasks/main.yaml

@@ -2,7 +2,7 @@
   when: zuul.change is defined
   delegate_to: localhost
   shell: |
-    if [ -n $(ls {{ zuul.executor.work_root }}/{{ item }}) ] ; then
+    if [ -n "$(find {{ zuul.executor.work_root }}/{{ item }} -mindepth 1)" ] ; then
       # Only create target directory if it is needed.
       # Do not fail if it is already there.
       mkdir -p {{ zuul.executor.log_root }}/{{ item }}