DS-440 #done Only return  if signup form instance

Fixes https://jira.dosomething.org/browse/DS-440

Stops the millions of "Notice: Undefined variable: forms in dosomething_signup_forms() (line 24 of /var/www/vagrant/lib/modules/dosomething/dosomething_signup/dosomething_signup.forms.inc)." messages I've flooded our logs with :(