Add isEmpty and deprecate null

[sigma-andex]

Description of the change

Adds a human-friendly isEmpty and deprecate null.

Related: https://github.com/purescript/purescript-arrays/pull/222

Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• [x] Linked any existing issues or proposals that this pull request should close
• [x] Updated or added relevant documentation
• [x] Added a test for the contribution (if applicable)