Support for Null Image input

[dtoconnor]

Feature request: allow messages with no images to be sent to the VLM (specifically Qwen2-VL-7B-8bit). This is supported with transformers and other libraries by default, but null images cause shape errors upon inference in MLX_VLM, e.g.

ValueError: Invalid high padding size (-578) passed to pad for axis 1. Padding sizes must be non-negative

Apart from that, I love the work you are doing.

[Blaizzy]

Hey @dtoconnor

Thanks, that's a nice suggestion

I will add it to the next release