SDL output for union with no types is invalid

[kdawgwilk]

Environment

• Elixir version (elixir -v): 1.10 or 1.11
• Absinthe version (mix deps | grep absinthe): 1.6.4 or master branch
• Client Framework and version (Relay, Apollo, etc): N/A

Expected behavior

union _Entity

Actual behavior

union _Entity = 

Relevant Schema/Middleware Code

In a schema processing phase, I am dynamically creating this union type and in some cases the types list is empty but when I ask Absinthe.Schema.to_sdl(MyApp.Schema) I get = in the output with no types on the other side. Tools like GraphiQL playground dont like this and throw errors

%UnionTypeDefinition{
      __reference__: Notation.build_reference(__ENV__),
      description: "a union of all types that use the @key directive",
      identifier: :_entity,
      module: __MODULE__,
      name: "_Entity",
      types: []
    }