`to_char(.., null)` should be NULL, not `..`

[alamb]

The newly added to_char's null handling is not quite correct -- specifically when the format string is null, the output should be null, but now it returns the first argument

Goal is to make to_char do what postgres does:

          Postgres seems to think this should be null

postgres=# select to_char(interval '1 day', null);
 to_char
---------

(1 row)

Originally posted by @alamb in https://github.com/apache/arrow-datafusion/pull/9181#discussion_r1487974457

[alamb]

There are existing tests in timestamp.slt, this should simply be a matter of updating the code (see #9181) -- I think this would be a good first issue

[manoj-inukolunu]

take

[tinfoil-knight]

@manoj-inukolunu Are you still working on this? (If not, could you please un-assign yourself. I'd like to attempt the fix for this issue)

[manoj-inukolunu]

Hey so sorry not working on it right now , swamped with work .

[manoj-inukolunu]

unassign

[tinfoil-knight]

take