Fix reasoning output in Responses

Author: Recouse

Sources/OpenAI/Responses/ResponsesResponse.swift

@@ -215,7 +215,7 @@ public extension Responses {
         /// turns of a conversation if you are manually managing context.
         public struct ReasoningOutput: Decodable, Sendable {
             /// Reasoning text content.
-            public let content: [Content]
+            public let content: [Content]?
 
             /// The encrypted content of the reasoning item - populated when a response is
             /// generated with `reasoning.encrypted_content` in the `include` parameter.