add null checks in type cast methods

[jennifersp]

No description provided.

[Hydrocharged]

We didn't do this for the normal functions, but instead of adding null checks to all of the cast functions, we should implement the actual null handling for functions.

https://www.postgresql.org/docs/15/sql-createfunction.html

In particular, CALLED ON NULL INPUT, RETURNS NULL ON NULL INPUT, and STRICT. We shouldn't necessarily do that on this PR though, since we also need to shift casts to the general function structure (like how we handle operators and comparisons). Instead, for this PR, since we know all of the casts use RETURNS NULL ON NULL INPUT (same as STRICT), we can simply add the null check before we call the cast function, so that it works for every cast.

For reference, this query looks at pg_proc for all cast functions, and we check the proisstrict column to determine whether a function is STRICT or not, so you can use this to check how any function is supposed to handle nulls.

SELECT pg_proc.* FROM pg_proc JOIN pg_cast ON pg_proc.oid = pg_cast.castfunc;

[Hydrocharged]

LGTM! Just need to readjust the location of the strict check