Nullif of NULL Predicate is not NULL

[tustvold]

Describe the bug

SQL NULLIF returns NULL if the predicate evaluates to NULL.

i.e.

> SELECT NULLIF(NULL, 1)
NULL

Whereas the nullif kernel in arrow preserves the nullability of the input.

This appears to date from the initial implementation of this kernel in https://github.com/apache/arrow/pull/8688/files#diff-d3f27fc6d1b5bf9dcc0eec10fb37f9f13e9dcdf5f608b932d0e0c0151a06b91aR253 by @velvia

To Reproduce

Expected behavior

I'm not really sure, I think it is surprising that we break SQL compatibility in this way, but we have been doing so for a while so I'm not sure if this is intentional or just nobody has noticed.

Additional context

[tustvold]

Looking at how this is typically used I realise the current logic is correct. In particular the common pattern is

let cond_array = eq_dyn(lhs, rhs)?;
// Now, invoke nullif on the result
let array = nullif(lhs, as_boolean_array(&cond_array)?)?;

In particular if lhs or rhs is null, cond_array will also be null.

The result should only be null if lhs is null, which is the case if NULL passed to nullif preserves the nullability of lhs in the event of rhs being null.

>>> duckdb.query("Select nullif(null, 1)").fetchall()
[(None,)]
>>> duckdb.query("Select nullif(true, 1)").fetchall()
[(None,)]
>>> duckdb.query("Select nullif(false, 1)").fetchall()
[(False,)]
>>> duckdb.query("Select nullif(null, null)").fetchall()
[(None,)]
>>> duckdb.query("Select nullif(null, 1)").fetchall()
[(None,)]
>>> duckdb.query("Select nullif(1, null)").fetchall()
[(1,)]

[alamb]

Posgres agrees:

postgres=# Select nullif(null, null);
 nullif 
--------
 
(1 row)

postgres=# Select nullif(null, 1)
postgres-# ;
 nullif 
--------
       
(1 row)

postgres=# Select nullif(1, null);
 nullif 
--------
      1
(1 row)

🤯