Implement Null Layout

Implement read and write of the [Null Layout](https://arrow.apache.org/docs/format/Columnar.html#null-layout) and verify against integration tests.

Start with the linked Apache Arrow Null Layout specification, then locate the existing layout read and write entry points in the Erlang implementation. Run the integration tests before and after the change; done means Null Layout data can be both read and written and the integration tests verify it.

Written by the indexing model from the issue text.