Fix 1581 by supporting null defaults for object and array

[heath-freenome]

Reasons for making this change

Fixed #1581 by fixing getDefaultFormState() to support null defaults for object and array types

• Updated getDefaultFormState() to add computeDefaultBasedOnSchemaTypeAndDefaults() to detect when ["null", "object"|"array"], default as null and the computed default was empty
  • Updated the tests for getDefaultFormState() to tests all of the computeDefaultBasedOnSchemaTypeAndDefaults()
• Updated CHANGELOG.md accordingly

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature