support is null or is not null checking inside case condition

Currently, null checking is only supported inside filtering. I want to support the following use case:

select case
when timeStamp IS NULL then now()
else timestamp
end
from meetupRsvp

No files, tests, or entry points are named. Start by locating the existing null-check handling used during filtering and the code that evaluates CASE conditions; add coverage for IS NULL and IS NOT NULL in a CASE WHEN expression, and verify the shown query produces the expected branch result.

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