Enable mapping to nil in enums

What does this MR do?

Enum in Rails 5 does not map nil values - IOW nil value remains nil, even if there is a key with nil value in the enum definition.

This commit overrides the underlying Enum methods so nil value is still mapped. This solution is far from being ideal: it uses dynamic definition of methods which introduces more magic/confusion into the codebase.

It would be better to get rid of the nil value in enums (which will be addressed in a follow-up issue).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47032 (closed), #46279 (closed)