AEP-126: HTTP APIs should not promote the use of the "null" value in enums.

[toumorokoshi]

Inlining a comment from @mkistler from slack:

I discovered this in AEP-126 while reviewing a PR to the OpenAPI linter for this AEP:
If the enum is optional, The null value should be used as the empty value, and should be the first value specified.
There is so much confusion about the meaning of null, and this guidance steps right into it.
I personally think this is bad guidance -- we should not promote using null to mean not provided or no value.

Filing here for posterity's sake.

[toumorokoshi]

I don't think this implies that null means "not provided" - but yes, it would be "no value": I don't believe those two are semantically equivalent, such as when "not provided" being a no-op vs an explicit null-ing of a value within a request.

I think there's a few different cases w.r.t. to an enum with an empty value:

1. A value that can indeed be null, in which case I think this is technically valid, in that a null value is valid?
2. A null value is not valid, but you don't have to set the value. In that case we should probably not include the value in the enum.

Perhaps the current wording doesn't clarify this enough with 2?