ServerVariable.enum should be optional

[Soremwar]

Per https://spec.openapis.org/oas/v3.1.0#fixed-fields-4, enum is An enumeration of string values to be used if the substitution options are from a limited set. The array MUST NOT be empty.

I think this was interpreted as enum being a required property, when in reality it's optional and treating it as such removes the ability for applications to set the specificied variables with an arbitrary value

Without enum:

With enum:

[luolingchun]

Thanks for your report, I will fix it.