arrow

MINOR: [Go] Add IsNull method to dictionary builder (#35956)

Given that this function is super similar to the previously added `GetValueIndex` I've decided to not open an Issue first.

### Rationale for this change

This is helpful to check if previously appended rows have been null or not.

### Are these changes tested?

The changes have been tested in our unit tests building on top of this. I'm happy to follow up with a unit test if deemed necessary. 

### Are there any user-facing changes?

The addition of `IsNull(i int) bool` to the `BinaryDictionaryBuilder`. 

Authored-by: Matthias Loibl <mail@matthiasloibl.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>