[Variant] Add `VariantArrayBuilder::append_nulls` API

[sdf-jkl]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

A comment by @scovich in #9598:
https://github.com/apache/arrow-rs/pull/9598/changes/BASE..4e3bbb38e5a5014ed1e9bf79df250e71e67b812b#r3058860532

There's been a recent bunch of PR adding (or expanding) support for bulk-appending NULL values to array builders, e.g. PrimitiveArrayBuilder::append_nulls, StructBuilder::append_nulls, etc. Should we take a follow-on item to add that support to variant array builder as well? (Some builders have append_values methods as well).

This will avoid for loops of VariantArrayBuilder::append_null.

Describe the solution you'd like

• Implement a new VariantArrayBuilder::append_nulls API

Describe alternatives you've considered

Additional context