Make VariantBean.gender optional and remove provisional enum value

[Luc1412]

Summary

This PR makes VariantBean.gender optional and removes the provisional enum value CustomGender.UNKNOWN introduced in 3.2.1.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)