Function calls with NULL literals do not work

[alamb]

Reproducer:

> select sqrt(null);
Plan("Coercion from [Utf8] to the signature Uniform(1, [Float64, Float32]) failed.")

The correct output is null

I think the solution will likely be to teach the function type coercion code about the null type (and that it can be cast to any other type). See #1184 for more details

[WinkerDu]

@alamb
I try to introduce ScalarValue::Null, which pub fn get_datatype(&self) -> DataType {..} method will return DataType::Null.
But for DataType::Utf8 type, it seems arrow-rs does not supported DataType::Null cast to DataType::Utf8 yet,
So I got panic error in some cases like Select ascii(NULL).

Or I just allow Utf8 coercion check pass for function NULL support?

What do you think about it ?

[alamb]

But for DataType::Utf8 type, it seems arrow-rs does not supported DataType::Null cast to DataType::Utf8 yet,

Aaah -- I remember hitting that (or something similiar) when I started messing around with null constants. We should probably start by fixing the cast kernel in arrow-rs to cast back/forth between null and any type. Is that something you would like to try or shall I try and find some time?

It actually seems as if @bjchambers suggested this last year apache/arrow-rs#684

[WinkerDu]

@alamb
I'm glad to have a try, and will do some research on it at first.
It will cost a few days since I'm quite busy this week.

[alamb]

Thanks @WinkerDu