max_tokens is None leads to internal server error

# Prerequisites

Please answer the following questions for yourself before submitting an issue.

- [X] I am running the latest code. Development is very rapid so there are no tagged versions as of now.
- [X] I carefully followed the [README.md](https://github.com/abetlen/llama-cpp-python/blob/main/README.md).
- [X] I [searched using keywords relevant to my issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/filtering-and-searching-issues-and-pull-requests) to make sure that I am creating a new issue that is not already open (or closed).
- [X] I reviewed the [Discussions](https://github.com/abetlen/llama-cpp-python/discussions), and have a new bug or useful enhancement to share.

# Expected Behavior

When a large query, comes in and max_tokens is not set but the model still fails with token limit exceeded this should be reflected in the error response.

# Current Behavior
When the input tokens are too big, in the error handling another error happens, due to line 209 in app.py which combines the `None` of `completion_tokens` with the `prompt_tokens`:
`completion_tokens + prompt_tokens`
Essentially completion_tokens should be checked for None in the function or set a non None value for the max_tokens number

# Environment and Context

Irrelevant

* Physical (or virtual) hardware you are using, e.g. for Linux:

Irrelevant

* Operating System, e.g. for Linux:

Irrelevant

* SDK version, e.g. for Linux:

Irrelevant

# Failure Information (for bugs)

Please help provide information about the failure if this is a bug. If it is not a bug, please remove the rest of this template.

# Steps to Reproduce

Spin up a llamacpp server
Send a request with more than ~2048 tokens without specifying a max_tokens parameter

# Failure Logs

```
INFO: 130.233.8.131:0 - "POST /v1/chat/completions HTTP/1.0" 500 Internal Server Error
ERROR: Exception in ASGI application
Traceback (most recent call last):
File "/usr/local/lib/python3.12/site-packages/llama_cpp/server/app.py", line 304, in custom_route_handler
response = await original_route_handler(request)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/fastapi/routing.py", line 274, in app
raw_response = await run_endpoint_function(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/fastapi/routing.py", line 191, in run_endpoint_function
return await dependant.call(**values)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/llama_cpp/server/app.py", line 852, in create_chat_completion
first_response = await run_in_threadpool(next, iterator_or_completion)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/starlette/concurrency.py", line 41, in run_in_threadpool
return await anyio.to_thread.run_sync(func, *args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/anyio/to_thread.py", line 33, in run_sync
return await get_asynclib().run_sync_in_worker_thread(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 877, in run_sync_in_worker_thread
return await future
^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 807, in run
result = context.run(func, *args)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/llama_cpp/llama_chat_format.py", line 222, in _convert_text_completion_chunks_to_chat
for i, chunk in enumerate(chunks):
File "/usr/local/lib/python3.12/site-packages/llama_cpp/llama.py", line 1415, in _create_completion
raise ValueError(
ValueError: Requested tokens (6692) exceed context window of 2048

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 408, in run_asgi
result = await app( # type: ignore[func-returns-value]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/fastapi/applications.py", line 1106, in __call__
await super().__call__(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette/applications.py", line 122, in __call__
await self.middleware_stack(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 184, in __call__
raise exc
File "/usr/local/lib/python3.12/site-packages/starlette/middleware/errors.py", line 162, in __call__
await self.app(scope, receive, _send)
File "/usr/local/lib/python3.12/site-packages/starlette/middleware/cors.py", line 83, in __call__
await self.app(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette_context/middleware/raw_middleware.py", line 92, in __call__
await self.app(scope, receive, send_wrapper)
File "/usr/local/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
raise exc
File "/usr/local/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
await self.app(scope, receive, sender)
File "/usr/local/lib/python3.12/site-packages/fastapi/middleware/asyncexitstack.py", line 20, in __call__
raise e
File "/usr/local/lib/python3.12/site-packages/fastapi/middleware/asyncexitstack.py", line 17, in __call__
await self.app(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette/routing.py", line 718, in __call__
await route.handle(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette/routing.py", line 276, in handle
await self.app(scope, receive, send)
File "/usr/local/lib/python3.12/site-packages/starlette/routing.py", line 66, in app
response = await func(request)
^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/llama_cpp/server/app.py", line 334, in custom_route_handler
) = self.error_message_wrapper(error=exc, body=body)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/llama_cpp/server/app.py", line 283, in error_message_wrapper
return callback(body, match)
^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/llama_cpp/server/app.py", line 209, in context_length_exceeded
completion_tokens + prompt_tokens,
~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

```