Handle null content for tool_calls assistant message for bedrock

[VisargD]

The following request object fails because tool_calls is not getting mapped if content is not present.

Example message object that fails:

{
            "role": "assistant",
            "content": null,
            "tool_calls": [
                {
                    "id": "tooluse_uzH92dzcSX6qfvub37M36A",
                    "type": "function",
                    "function": {
                        "name": "calculator",
                        "arguments": "{\"input\":\"128491981235*2152315135\"}"
                    }
                }
            ]
 }

Error message:

{
    "error": {
      "message": "bedrock error: The content field in the Message object at messages.[position] is empty. Add a ContentBlock object to the content field and try again.",
      "type": null,
      "param": null,
      "code": null
    },
    "provider": "bedrock"
}