Cryptic warning when generating an input with not fields

[rgraff]

Describe the bug
When you have an input resource with no fields, you get a cryptic warning:

warning: Embedded type MyApp.SomeUnionType.SomeSubType does not have an input type defined, but is accepted as input in at least one location.
  (ash_graphql 1.3.2) lib/resource/resource.ex:4296: AshGraphql.Resource.do_field_type/5
  (ash_graphql 1.3.2) lib/resource/resource.ex:3293: anonymous fn/4 in AshGraphql.Resource.union_type_definitions/6
  (elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2
  (ash_graphql 1.3.2) lib/resource/resource.ex:3287: AshGraphql.Resource.union_type_definitions/6
  (elixir 1.17.2) lib/enum.ex:4353: Enum.flat_map_list/2
  (elixir 1.17.2) lib/enum.ex:4354: Enum.flat_map_list/2

To Reproduce
In my particular instance, I was scaffolding out a new UnionType and its various SubTypes but I hadn't defined any attributes yet and had an empty accepts

Expected behavior
A warning that says it can't define an input type without at least one input attribute.

** Runtime

• Elixir version
• Erlang version
• OS
• Ash version
• any related extension versions

Additional context
Add any other context about the problem here.