fix: allow empty content for Response (open-api-spex#453)

lib/open_api_spex/operation.ex

@@ -139,8 +139,8 @@ defmodule OpenApiSpex.Operation do
   """
   @spec response(
           description :: String.t(),
-          media_type :: String.t() | %{String.t() => Keyword.t() | MediaType.t()},
-          schema_ref :: Schema.t() | Reference.t() | module,
+          media_type :: String.t() | %{String.t() => Keyword.t() | MediaType.t()} | nil,
+          schema_ref :: Schema.t() | Reference.t() | module | nil,
           opts :: keyword
         ) :: Response.t()
   def response(description, media_type, schema_ref, opts \\ []) do
@@ -158,7 +158,7 @@ defmodule OpenApiSpex.Operation do
     %Response{
       description: description,
       headers: opts[:headers],
-      content: build_content_map(media_type, content_opts)
+      content: build_response_content_map(media_type, content_opts)
     }
   end
 
@@ -303,6 +303,11 @@ defmodule OpenApiSpex.Operation do
     |> Schema.validate(params, schemas)
   end
 
+  defp build_response_content_map(nil, _media_type_opts), do: nil
+
+  defp build_response_content_map(media_type, media_type_opts),
+    do: build_content_map(media_type, media_type_opts)
+
   defp build_content_map(media_type, media_type_opts) when is_binary(media_type) do
     %{
       media_type => struct!(MediaType, media_type_opts)
@@ -317,6 +322,6 @@ defmodule OpenApiSpex.Operation do
   end
 
   defp build_content_map(media_types, _shared_opts) do
-    raise "Expected string or map for request_body: #{inspect(media_types)}"
+    raise "Expected string or map as a media type. Got: #{inspect(media_types)}"
   end
 end