Adding an input_object without fields causes schema not to generate and fail silently

[MikaAK]

Environment

• Elixir version (elixir -v): 1.13.3
• Absinthe version (mix deps | grep absinthe): 1.6.6

When generating the API if you specify an input_object with no fields:

input_object :my_input do
end

The schema will no longer generate but there's no errors so the only way to track it down is start removing queries/mutations to see what generates

Expected behavior

Probably some sort of error would be nice saying there's an empty schema

Actual behavior

Schema just doesn't generate and there's an empty schema in /graphiql

Relevant Schema/Middleware Code

mutation do
  field :hello_world, :string do 
    arg :input, :my_input
  end
end

input_object :my_input do
end