ImageFolder raises an error with parameters drop_metadata=True and drop_labels=False when metadata.jsonl is present
## Describe the bug

If you pass `drop_metadata=True` and `drop_labels=False` when a `data_dir` contains at least one `matadata.jsonl` file, you will get a KeyError. This is probably not a very useful case but we shouldn't get an error anyway. Asking users to move metadata files manually outside `data_dir` or pass features manually (when there is a tool that can infer them automatically) don't look like a good idea to me either.

## Steps to reproduce the bug
### Clone an example dataset from the Hub
```bash
git clone https://huggingface.co/datasets/nateraw/test-imagefolder-metadata
```
### Try to load it
```python
from datasets import load_dataset
ds = load_dataset("test-imagefolder-metadata", drop_metadata=True, drop_labels=False)
```
or even just
```python
ds = load_dataset("test-imagefolder-metadata", drop_metadata=True)
```
as `drop_labels=False` is a default value.

## Expected results
A DatasetDict object with two features: `"image"` and `"label"`.

## Actual results
```
Traceback (most recent call last):
  File "/home/polina/workspace/datasets/debug.py", line 18, in <module>
    ds = load_dataset(
  File "/home/polina/workspace/datasets/src/datasets/load.py", line 1732, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/polina/workspace/datasets/src/datasets/builder.py", line 704, in download_and_prepare
    self._download_and_prepare(
  File "/home/polina/workspace/datasets/src/datasets/builder.py", line 1227, in _download_and_prepare
    super()._download_and_prepare(dl_manager, verify_infos, check_duplicate_keys=verify_infos)
  File "/home/polina/workspace/datasets/src/datasets/builder.py", line 793, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/polina/workspace/datasets/src/datasets/builder.py", line 1218, in _prepare_split
    example = self.info.features.encode_example(record)
  File "/home/polina/workspace/datasets/src/datasets/features/features.py", line 1596, in encode_example
    return encode_nested_example(self, example)
  File "/home/polina/workspace/datasets/src/datasets/features/features.py", line 1165, in encode_nested_example
    {
  File "/home/polina/workspace/datasets/src/datasets/features/features.py", line 1165, in <dictcomp>
    {
  File "/home/polina/workspace/datasets/src/datasets/utils/py_utils.py", line 249, in zip_dict
    yield key, tuple(d[key] for d in dicts)
  File "/home/polina/workspace/datasets/src/datasets/utils/py_utils.py", line 249, in <genexpr>
    yield key, tuple(d[key] for d in dicts)
KeyError: 'label'
```

## Environment info
`datasets` master branch 

- `datasets` version: 2.3.3.dev0
- Platform: Linux-5.14.0-1042-oem-x86_64-with-glibc2.17
- Python version: 3.8.12
- PyArrow version: 6.0.1
- Pandas version: 1.4.1

