Which part is this question about
API of arrow_array::builder.

Describe your question
BooleanBuilder and PrimitiveBuilder provide read-only access to their null buffer via a validity_slice() method. However, GenericListBuilder does not provided such access, even though it does expose list offsets via the offsets_slice() method.

Is there a reason why this is the case, or may I submit a PR to add validity_slice() methods to this builder, and possibly other builders with a null buffer along the way?

Additional context
In the context of #5700, I'm investigating which strongly typed builders could expose their contents via a generalized slice type. Right now, I would able to do so for builders of non-nullable lists, but not for builders of nullable lists, due to lack of access to the list builder's null buffer. Exposing offsets but not the null buffer seems odd, hence my asking.