Fix for bugzid:99840 - Signup via Facebook broken

The code was trying to validate fields that would appear only after
Facebook would invoke a callback breaking the JS; the way the validation
class is implemented doesn't seem to allow for attaching fields to
validate in a second moment (those need to be passed to the constructor)
and those fields (username
and password) are validated server-side so for the moment we're covered
despite detecting empty fields would be a better thing to do client-side
as it happens for the standard signup form.

There was also a small bug in the validator class which would still try
to verify a captcha field when none was passed to the constructor
(Facebook signup doesn't include captcha's, we trust Facebook).

@kvas-damian @sebastian-wikia : this will fix the problem for now but
I'd suggest to review the implementatio to fit in the Facebook case so
that we won't force a full roundtrip for checking empty fields.

Thanks.