[v4] Enum sanitise allows the result to be empty string

[MasonD]

An empty string is not a valid graphql enum, but if a developer uses a weird enum value for a dataobject (say, '' or '-'), then sanitise will be fine with chomping that entirely down to the empty string '' and returning that. This doesn't cause any server-side errors and the graphql server will happily compile and serve responses, even responding to introspection queries.

But an empty enum value isn't valid graphql and this can cause interop problems with clients and developer tooling such as graphql-codegen.