don't fail when no groups are defined for user

tasks/main.yml

@@ -20,7 +20,7 @@
   user:
     name: "{{ item }}"
     group: "{{ item }}"
-    groups: "{{ users[item].groups | join(',') }}"
+    groups: "{{ users[item].groups | default([]) | join(',') }}"
     uid: "{{ users[item].uid }}"
     home: "{{ users[item].home | default('/home/'+item) }}"
     shell: "{{ users[item].shell | default('/bin/bash') }}"