Improve support for Null values in DataFrame GroupBy

[pgovind]

At the moment, null values are considered equivalent to default. Instead, null should be considered its own group

[pgovind]

dotnet/corefxlab#2701 implements an immutable arrow string compatible column. The GroupBy method there creates a temp group called "null" for now. That needs to be fixed too.

[pgovind]

Fixing this will also allow for optimizations in GroupBy.cs. At the moment, there are checks in some of the functions to account for the null.