feat: unset user properties when variant evaluates to null or is a fa…

[curtisliu]

…llback

Summary

This adds the ability to unset user properties when a variant has no value or is a fallback.

Other changes:

• Fallback order changed for source = InitialVariants:
  1. InitialFlags
  2. Local Storage
  3. Function fallback
  4. Config fallback

Checklist

• [x] Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

[github-actions[bot]]

:tada: This PR is included in version 1.4.0 :tada:

The release is available on:

• GitHub release
• v1.4.0

Your semantic-release bot :package::rocket: