zypper: fix for checking result is None (#3143)

lib/ansible/modules/extras/packaging/os/zypper.py

@@ -267,6 +267,7 @@ def get_cmd(m, subcommand):
 def set_diff(m, retvals, result):
     # TODO: if there is only one package, set before/after to version numbers
     packages = {'installed': [], 'removed': [], 'upgraded': []}
+    if result:
         for p in result:
             group = result[p]['group']
             if group == 'to-upgrade':