Casts from jsonb to other types should cope with json null

I complained in the discussion of bug #18564 [1] that it's quite
inconsistent that you can cast a jsonb null to text and get
a SQL NULL:

=# select ('{"a": null}'::jsonb)->>'a';
?column?
----------

(1 row)

but if you cast it to any other type it's an error:

=# select (('{"a": null}'::jsonb)->'a')::float8;
ERROR: cannot cast jsonb null to type double precision

I think this should be allowed and should produce a SQL NULL.
It doesn't look hard: the attached POC patch fixes this for
the float8 case only. If there's not conceptual objections
I can flesh this out to cover the other jsonb-to-XXX
cast functions.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/18564-5985f90678ed7512%40postgresql.org