fix(examples): handle usage chunk in tool call streaming (#1068)

---------

Co-authored-by: Jacob Zimmerman <jacob@stainlessapi.com>

examples/tool-calls-stream.ts

@@ -184,7 +184,13 @@ function messageReducer(previous: ChatCompletionMessage, item: ChatCompletionChu
     }
     return acc;
   };
-  return reduce(previous, item.choices[0]!.delta) as ChatCompletionMessage;
+
+  const choice = item.choices[0];
+  if (!choice) {
+    // chunk contains information about usage and token counts
+    return previous;
+  }
+  return reduce(previous, choice.delta) as ChatCompletionMessage;
 }
 
 function lineRewriter() {