Add coercion support for NULL literals

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11851

As we observed in apache/arrow#9565 (comment) datafusion won't coerce null literals, forcing strange syntax such as:

rpad('hi', CAST(NULL AS INT), 'xy')

We should add automatic coercion logic from the null literal to any type and this expression should work just fine (produce a NULL output)

rpad('hi', NULL, 'xy')

[alamb]

Comment from Mike Seddon(MikeSeddonAU) @ 2021-03-03T21:52:10.241+0000:

[~alamb] Previously I implemented the Null=UTF8 type based on the behavior of Postgres not Spark - because the Spark implementation comes with a lot of side effects like the fact that Parquet does not support the `NullType`. We can revisit.

Comment from Andrew Lamb(alamb) @ 2021-03-04T19:29:54.556+0000:

Makes sense to me

[alamb]

This appears to work just fine now:

❯ select cast(null as int);
+------+
| NULL |
+------+
|      |
+------+
1 row in set. Query took 0.033 seconds.

🎉