This check looks like it does not belong here, here we see "PluginOutput" and "Plugin", but not the options supplied by the user, for aux plugins you should check if the parameters supplied by the user matched those defined in the aux plugin.
Aux plugins define their parameters like this:
https://github.com/owtf/owtf/blob/master/plugins/aux/bruteforce/Password_Bruteforce%40OWTF-ABrF-001.py#L39

Note the "optional" and "mandatory" there, that is what you need to enforce: "mandatory" parameters should be supplied by the user, and "optional" should be defaulted by owtf when not supplied.