purge_instance_history_by raises exception when optional created_time_from is not passed

🐛 **Describe the bug**
> A clear and concise description of what the bug is.

When `created_time_from: Optional[datetime] = None` is not passed to `purge_instance_history_by` it causes an exception in `delete_async_request`.


```
Exception has occurred: Exception
The operation failed with an unexpected status code 400
File "/home/andre/code/durable-function/.venv/lib/python3.10/site-packages/azure/durable_functions/models/DurableOrchestrationClient.py", line 413, in purge_instance_history_by
return self._parse_purge_instance_history_response(response)
File "/home/andre/code/durable-function/function_app.py", line 15, in http_clean
result = await client.purge_instance_history_by(
File "/home/andre/code/durable-function/.venv/lib/python3.10/site-packages/azure/durable_functions/decorators/durable_app.py", line 199, in df_client_middleware
return await user_code(*args, **kwargs)
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/azure_functions_worker/extension.py", line 147, in get_async_invocation_wrapper
result = await function(**args)
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/azure_functions_worker/dispatcher.py", line 758, in _run_async_func
return await ExtensionManager.get_async_invocation_wrapper(
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/azure_functions_worker/dispatcher.py", line 475, in _handle__invocation_request
call_result = await self._run_async_func(
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/azure_functions_worker/dispatcher.py", line 261, in _dispatch_grpc_request
resp = await request_handler(request)
File "/usr/lib/python3.10/asyncio/events.py", line 80, in _run
self._context.run(self._callback, *self._args)
File "/usr/lib/python3.10/asyncio/base_events.py", line 1909, in _run_once
handle._run()
File "/usr/lib/python3.10/asyncio/base_events.py", line 603, in run_forever
self._run_once()
File "/usr/lib/python3.10/asyncio/base_events.py", line 636, in run_until_complete
self.run_forever()
File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run
return loop.run_until_complete(main)
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/azure_functions_worker/main.py", line 49, in main
return aio_compat.run(start_async(
File "/usr/lib/azure-functions-core-tools-4/workers/python/3.10/LINUX/X64/worker.py", line 74, in
main.main()
File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
return _run_code(code, main_globals, None,
Exception: The operation failed with an unexpected status code 400
```

🤔 **Expected behavior**
> What should have happened?

Clean instances older than `created_time_to`. Or `purge_instance_history_by` not to allow `None` in `created_time_to`.

☕ **Steps to reproduce**
> What Durable Functions patterns are you using, if any?

> Any minimal reproducer we can use?

Yes. function_app.py below replicates the issue.

```python3
import azure.functions as func
import azure.durable_functions as df
from datetime import datetime, timedelta

app = df.DFApp(http_auth_level=func.AuthLevel.ANONYMOUS)

@app.route(route="clean")
@app.durable_client_input(client_name="client")
async def http_clean(req: func.HttpRequest, client: df.DurableOrchestrationClient):
now = datetime.today()
created_time_to = now + timedelta(minutes=1) # leave at least 1 mins

# Clean up function
result = await client.purge_instance_history_by(
created_time_to=created_time_to,
)
return f"Cleaned {result.instances_deleted}"
```

For example if only `created_time_from` is passed, it works as expected and the entries are deleted but if only `created_time_to` is passed, it throws the 400 exception.
```python3
created_time_from = now - timedelta(hours=50)
# Clean up function
result = await client.purge_instance_history_by(
created_time_from=created_time_from,
)
```

> Are you running this locally or on Azure?

Locally.

```
azure-functions==1.17.0
azure-functions-durable==1.2.8
```

⚡**If deployed to Azure**
> We have access to a lot of telemetry that can help with investigations. Please provide as much of the following information as you can to help us investigate!

- **Timeframe issue observed**:
- **Function App name**:
- **Function name(s)**:
- **Azure region**:
- **Orchestration instance ID(s)**:
- **Azure storage account name**:

> If you don't want to share your Function App or storage account name GitHub, please at least share the orchestration instance ID. Otherwise it's extremely difficult to look up information.

从 azure/durable_functions/models/DurableOrchestrationClient.py 中的 purge_instance_history_by 开始，重点检查在第 413 行附近到达 delete_async_request 的路径。使用仅包含 created_time_to 的 function_app.py 示例重现该问题，然后将其与正常工作的 created_time_from 情况进行比较。当仅使用 created_time_to 进行清理时不再返回报告的 400 错误，即表示完成。

由索引模型根据 Issue 内容生成。