Fix handling of "undef" in contrib/jsonb_plperl.

Perl has multiple internal representations of "undef", and just
testing for SvTYPE(x) == SVt_NULL doesn't recognize all of them,
leading to "cannot transform this Perl type to jsonb" errors.
Use the approved test SvOK() instead.

Report and patch by Ivan Panchenko.  Back-patch to v11 where
this module was added.

Discussion: https://postgr.es/m/1564783533.324795401@f193.i.mail.ru