Fix case evaluation with NULL

[jonahgao]

Which issue does this PR close?

Closes #6376.

Rationale for this change

As pointed out by @richox on #6376

zip() does not specify the behavior of null input and it doesn't check null values in its implementation.

What changes are included in this PR?

Call the prep_null_mask_filter function to eliminate NULLs.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[alamb]

LGTM -- thank you @byteink

[alamb]

👍 matches postgres:

postgres=# select case when a = 0 then 123 end from (values(1), (0), (null)) as t(a);
 case
------

  123

(3 rows)