[ARROW-11851] [Rust][DataFusion] Add coercion support for `NULL` literals

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Invalid
• Component/s: Rust - DataFusion
• External issue URL: https://github.com/apache/arrow/issues/27698
• Assignee: Unassigned
• Reporter: Andrew Lamb
• Created: 03/Mar/21 11:49
• Updated: 11/Jan/23 08:22
• Resolved: 26/Apr/21 13:31

Description

As we observed in https://github.com/apache/arrow/pull/9565#discussion_r586347165 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')
```