pgsql: Make jsonb casts to scalar types translate JSON null to SQL NULL

Make jsonb casts to scalar types translate JSON null to SQL NULL.

Formerly, these cases threw an error "cannot cast jsonb null to type
<whatever>". That seems less than helpful though. It's also
inconsistent with the behavior of the ->> operator, which translates
JSON null to SQL NULL, as do some other jsonb functions.

Discussion: https://postgr.es/m/3851203.1722552717@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a5579a90af05814eb5dc2fd5f68ce803899d2504

Modified Files
--------------
src/backend/utils/adt/jsonb.c | 77 +++++++++++++++++++++++++++++++++----
src/test/regress/expected/jsonb.out | 66 +++++++++++++++++++++++++++++++
src/test/regress/sql/jsonb.sql | 13 +++++++
3 files changed, 149 insertions(+), 7 deletions(-)