Add linter rule or http warning for empty scopes

[MaryGao]

The scopes for AuthorizationCodeFlow is required but allowed for empty values. But actually an empty scope is not acceptable in Azure SDK. Could we add a linter to warn the empty scopes?

https://github.com/microsoft/typespec/blob/main/packages/http/lib/auth.tsp#L148

See dicussions Azure/azure-rest-api-specs#26038 (comment)