panic when case statement returns null

[kmitchener]

Describe the bug
A clear and concise description of what the bug is.

I've boiled this down to a small test case which panics with this error:
thread 'main' panicked at 'NullArray data should not contain a null buffer, as no buffers are required'

To Reproduce
Steps to reproduce the behavior:

run this SQL from cli:
select case when b is null then null else b end from (select a,b from (values (1,null),(2,3)) as t (a,b)) a;

Expected behavior
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In Postgres, it returns "null" and 3, as expected.

Additional context
Add any other context about the problem here.

Also, this query returns wrong results, as compared to PG:
select case when b is null then null else b end from (select a,b from (values (1,1),(2,3)) as t (a,b)) a;

in PG, that returns 2 rows: 1 and 3. In Datafusion, it returns 2 nulls.

[rostamn739]

also happens with
select case when b is null then null else b end from (select a,b from (values (1,1),(2,null)) as t (a,b)) a;

but does not happen with
select case when b is null then null else 1 end from (select a,b from (values (1,null),(2,3)) as t (a,b)) a;