[Rust][DataFusion] Add coercion support for NULL literals

[asfimport]

As we observed in #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')

**Reporter**: [Andrew Lamb](https://issues.apache.org/jira/browse/ARROW-11851) / @alamb

<sub>**Note**: *This issue was originally created as [ARROW-11851](https://issues.apache.org/jira/browse/ARROW-11851). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*</sub>

[asfimport]

Mike Seddon / @seddonm1:
@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.

[asfimport]

Andrew Lamb / @alamb:
Makes sense to me

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/datafusion#185