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MINOR: [Go] DictionaryBuilder reads null bitmap from idxBuilder (#36165)

This updates the `dictionaryBuilder` to read the idxBuilder's nullBitmap such that it not only panics but reads the correct NULLs. 

### Rationale for this change

Fix the method for the dictionaryBuilder. 

### What changes are included in this PR?

This updates the `dictionaryBuilder` to read the idxBuilder's nullBitmap such that it not only panics but reads the correct NULLs. 

### Are these changes tested?

Yes, updated the unit test.

### Are there any user-facing changes?

No

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