Introduce `ScalarValue::Null` type to df

[WinkerDu]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
To solve Null constants issues listed in #1184 , and since apache/arrow-rs#1572 supports Null casted from and to most of types in arrow-rs kernel, it's reasonable that introduce Null type to df for type coercion.

Describe the solution you'd like
Introduce ScalarValue::Null type to df

Describe alternatives you've considered
No.

Additional context
No.