pgsql: Fix handling of "undef" in contrib/jsonb_plperl.

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/df521ab79547b82471126e54e9dc7fead4a5a4fb

Modified Files
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contrib/jsonb_plperl/expected/jsonb_plperl.out  | 22 +++++++++++++++++++++-
contrib/jsonb_plperl/expected/jsonb_plperlu.out | 22 +++++++++++++++++++++-
contrib/jsonb_plperl/jsonb_plperl.c             | 10 +++++-----
contrib/jsonb_plperl/sql/jsonb_plperl.sql       | 13 +++++++++++++
contrib/jsonb_plperl/sql/jsonb_plperlu.sql      | 13 +++++++++++++
5 files changed, 73 insertions(+), 7 deletions(-)