Render null default values in SDL

lib/absinthe/language/input_value_definition.ex

@@ -48,6 +48,9 @@ defmodule Absinthe.Language.InputValueDefinition do
     defp to_term(%Language.ListValue{values: values}),
       do: Enum.map(values, &to_term/1)
 
+    defp to_term(%Language.NullValue{}),
+      do: nil
+
     defp to_term(%Language.ObjectValue{fields: fields}),
       do: Enum.into(fields, %{}, &{String.to_atom(&1.name), to_term(&1.value)})
 

lib/absinthe/schema/notation/sdl_render.ex

@@ -408,6 +408,9 @@ defmodule Absinthe.Schema.Notation.SDL.Render do
   defp render_value(%Blueprint.Input.Value{raw: raw}),
     do: render_value(raw)
 
+  defp render_value(%Blueprint.Input.Null{}),
+    do: "null"
+
   defp render_value(%Blueprint.Input.Object{fields: fields}) do
     default_fields = Enum.map(fields, &render_value/1)
     concat(["{", join(default_fields, ", "), "}"])

test/absinthe/schema/sdl_render_test.exs

@@ -53,6 +53,7 @@ defmodule Absinthe.Schema.SdlRenderTest do
       defaultInputArg(input: ComplexInput = {foo: "bar"}): String
       defaultListArg(things: [String] = ["ThisThing"]): [String]
       defaultEnumArg(category: Category = NEWS): Category
+      defaultNullStringArg(name: String = null): String
       animal: Animal
     }