`SplitDict.copy()` crashes when `dataset_name` is not set

## Description

`SplitDict.copy()` (and `SplitDict.from_split_dict`) crashes with
`AttributeError: 'SplitInfo' object has no attribute 'get'` whenever the split
dict contains `SplitInfo` objects and `dataset_name` is `None`.

## Reproduction

```python
from datasets import SplitDict, SplitInfo

split_dict = SplitDict({"train": SplitInfo(name="train", num_bytes=1337, num_examples=42)})
split_dict.copy()
```

Traceback:

```
File ".../datasets/splits.py", line 574, in from_split_dict
dataset_name = split_infos[0].get("dataset_name") if split_infos else None
AttributeError: 'SplitInfo' object has no attribute 'get'
```

## Expected behavior

`copy()` should work for `SplitDict` instances holding `SplitInfo` objects,
which is the normal state after construction (e.g. from a dataset's info).
The method should read `dataset_name` from `SplitInfo.dataset_name` when the
entries are `SplitInfo` objects, and only fall back to `.get()` for raw
dictionaries.

## Environment

- `datasets` version: 5.0.2.dev0 (main, d336dcb)
- Python: 3.11

Start in datasets/splits.py around SplitDict.from_split_dict, referenced at line 574, and trace how SplitInfo entries and raw dictionaries are handled. Run the reproduction from the issue, then verify that SplitDict.copy() and SplitDict.from_split_dict work with SplitInfo objects when dataset_name is unset and preserve the expected dataset name when it is available.

Written by the indexing model from the issue text.