Fix overly strict check that was breaking the null case.

Author: Manouchehri

packages/agents-openai/src/openaiChatCompletionsModel.ts

@@ -67,7 +67,7 @@ export class OpenAIChatCompletionsModel implements Model {
     const output: protocol.OutputModelItem[] = [];
     if (response.choices && response.choices[0]) {
       const message = response.choices[0].message;
-      if (message.content !== undefined) {
+      if (message.content) {
         const { content, ...rest } = message;
         output.push({
           id: response.id,