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
".  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