translations: Check for translation-source first

Translation jobs might not produce the translation-source directory but
still succeed, e.g. if the branch is not translated.

Check for the translation-source directory and only fetch files if there
is really something to fetch.

Change-Id: I064873412bf8503c5e7d317739adb891315038f8

roles/fetch-translation-output/tasks/main.yaml

@@ -1,6 +1,12 @@
+- name: Check for translation files
+  stat:
+    path: "{{ translation_output_src }}"
+  register: translation_files
+
 - name: Collect translation source files
   synchronize:
     dest: "{{ zuul_executor_dest }}"
     mode: pull
     src: "{{ translation_output_src }}"
     verify_host: true
+  when: translation_files.stat.exists