Support chat scenarios without images

[madroidmaq]

The current code implementation does not support the case of image == None, I'm not sure if this was intentionally designed this way or if this scenario was forgotten.

Below is the test code:

import requests
import json

url = "http://localhost:8000/v1/chat/completions"
headers = {"Content-Type": "application/json"}
data = {
    "model": "mlx-community/nanoLLaVA-1.5-4bit",
    # "image": "http://images.cocodataset.org/val2017/000000039769.jpg",
    "messages": [{"role": "user", "content": "What are these"}],
    "max_tokens": 100
}

response = requests.post(url, headers=headers, data=json.dumps(data))
print(response.json())

error

Traceback (most recent call last):
  File "/opt/homebrew/lib/python3.11/site-packages/uvicorn/protocols/http/httptools_impl.py", line 426, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 65, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 756, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 776, in app
    await route.handle(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 297, in handle
    await self.app(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 77, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/homebrew/lib/python3.11/site-packages/starlette/routing.py", line 72, in app
    response = await func(request)
               ^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/routing.py", line 278, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastapi/routing.py", line 191, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/fastmlx/fastmlx.py", line 160, in chat_completion
    output = vlm_generate(
             ^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mlx_vlm/utils.py", line 1040, in generate
    inputs = prepare_inputs(
             ^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mlx_vlm/utils.py", line 770, in prepare_inputs
    pixel_values = image_processor.preprocess(images=images)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mlx_vlm/models/llava_bunny/llava_bunny.py", line 85, in preprocess
    images = reduce(lambda x, f: [*map(f, x)], transforms, images)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mlx_vlm/models/llava_bunny/llava_bunny.py", line 85, in <lambda>
    images = reduce(lambda x, f: [*map(f, x)], transforms, images)
                                 ^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/transformers/image_utils.py", line 213, in to_numpy_array
    raise ValueError(f"Invalid image type: {type(img)}")
ValueError: Invalid image type: <class 'NoneType'>