BUILD: cli: appease a null-deref warning in cli_gen_usage_msg()

BUILD: cli: appease a null-deref warning in cli_gen_usage_msg()

The compiler sees the possibility of null-deref for which a path is
possible but which doesn't exist as we didn't pass a null args outside
of the help request. The test was introduced by the simplified test on
ishelp variable, so let's add it to shut the warning.