fixed disappearing groups on OSX user module

Ensure that we don't try to modify the groups collection if groups are not specified

lib/ansible/modules/system/user.py

@@ -1673,6 +1673,7 @@ class DarwinUser(User):
         self._update_system_user()
         # here we don't care about change status since it is a creation,
         # thus changed is always true.
+        if self.groups:
             (rc, _out, _err, changed) = self._modify_group()
             out += _out
             err += _err
@@ -1707,6 +1708,7 @@ class DarwinUser(User):
             err += _err
             changed = rc
 
+        if self.groups:
             (rc, _out, _err, _changed) = self._modify_group()
             out += _out
             err += _err