Add a check in `ImageToTextPipeline._forward`
# What does this PR do?

Inside `ImageToTextPipeline.preprocess`, we have 
```python
        if self.model.config.model_type == "git" and prompt is None:
            model_inputs["input_ids"] = None
```
So it may  happen with a list of `None` (for Git model), and the `_forward` fail.

This PR add a check and change the above case to a single `None` value to avoid failure.
_The documentation is not available anymore as the PR was closed or merged._