Add NullState::is_null public method

[joroKr21]

Is your feature request related to a problem or challenge?

For some group accumulators it would be necessary to check if the previous value was null explicitly. E.g. consider implementing GroupsAccumulator for FirstValue. One could explicitly store the null state of all groups but if we already use NullState it would make sense to reuse it for null checking as well.

Describe the solution you'd like

Add a public NullState::is_null method that takes and index and returns bool.

Describe alternatives you've considered

The alternative is to store a duplicate null state (e.g. as a NullBufferBuilder) in the groups accumulator but that is more wasteful.

Additional context

No response

[joroKr21]

This method would be unusable because the callback-based APIs of NullState borrow it as mutable