[JENKINS-73690] Lack of support for enum being null (throwing exception)

[Wadeck]

Jenkins and plugins versions report

Environment

Paste the output here

What Operating System are you using (both controller, and any agents involved in the problem)?

N/A

Reproduction steps

When a Stapler WebMethod is defined with an Enum, the value cannot be null, otherwise it's throwing a NPE in the Converter code.

Expected Results

No exception, but a variable being null. With respect to the required=false.

Actual Results

NPE

Anything else?

JENKINS-73690

Are you interested in contributing a fix?

#577