text: Title: Using load_dataset with data_files and split arguments yields an error
  Body: ### Describe the bug
  
  It seems the list of valid splits recorded by the package becomes incorrectly overwritten when using the `data_files` argument.
  
  If I run
  ```python
  from datasets import load_dataset
  load_dataset("allenai/super", split="all_examples", data_files="tasks/expert.jsonl")
  ```
  then I get the error
  ```
  ValueError: Unknown split "all_examples". Should be one of ['train'].
  ```
  
  However, if I run
  ```python
  from datasets import load_dataset
  load_dataset("allenai/super", split="train", name="Expert")
  ```
  then I get
  ```
  ValueError: Unknown split "train". Should be one of ['all_examples'].
  ```
  
  ### Steps to reproduce the bug
  
  Run
  ```python
  from datasets import load_dataset
  load_dataset("allenai/super", split="all_examples", data_files="tasks/expert.jsonl")
  ```
  
  ### Expected behavior
  
  No error.
  
  ### Environment info
  
  Python = 3.12
  datasets = 3.2.0