Handle function call messages with empty content

[gilbertl]

When using OpenAI, assisnt reply tool-call like so

{
  "role": "assistant",
  "content": null,
  "tool_calls": [...]
}

When dealing with images, Azure OpenAI chat completions endpoint return an empty string instead of null

(example)

{
  "role": "assistant",
  "content": "",
  "tool_calls": [
    {
      "id": "call_RoAWzCh7KdKkR3LhmA9H0hqN",
      "type": "function",
      "function": {
        "name": "verify_invoice_number",
        "arguments": "{\"query\":\"74012938475019283746502391\"}"
      }
    }
  ]
}

Because the parser rejects this shape, the SDK:

1. treats the message as an ordinary assistant response,
2. never invokes the tool’s execute() handler,
3. asks the model to “try again,” and
4. eventually throws “Max turns (N) exceeded.”

Reproduction steps

1. Create any agent with a simple tool (e.g., check_image_color).2
2. set OpenAI client but with Azure as the endpoint
3. Call await run(agent, [{ role: "user", content: }])

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1de4760

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[gilbertl]

i noticed the change caused a test failure. Made a new commit and now the test suite passes.