Make SMTP username and password optional

[kuya1284]

There are people like me who may have a local SMTP server (i.e. Postfix) running that doesn't require authentication. Thus, both WUD_TRIGGER_SMTP_{trigger_name}_USER and WUD_TRIGGER_SMTP_{trigger_name}_PASS really should be optional.

As an interim solution, I just set both to some random string (i.e. null).