Trainer.repo.push_to_hub returns None, causing raised exception
### System Info

- `transformers` version: 4.28.1
- Platform: Windows-10-10.0.22621-SP0
- Python version: 3.11.2
- Huggingface_hub version: 0.14.1
- Safetensors version: not installed
- PyTorch version (GPU?): 2.0.1+cu117 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: NO
- Using distributed or parallel set-up in script?: NO

### Who can help?

@sgugger

### Information

- [X] The official example scripts
- [X] My own modified scripts

### Tasks

- [X] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

For some root cause that I'm not certain of, `Trainer.repo.push_to_hub` can return `None`, which causes `Trainer._push_from_checkpoint` to raise an exception (as it expects a tuple to be returned).

```
Traceback (most recent call last):
  File "F:\eo-reco\run_speech_recognition_ctc.py", line 810, in <module>
    main()
  File "F:\eo-reco\run_speech_recognition_ctc.py", line 756, in main
    train_result = trainer.train(resume_from_checkpoint=checkpoint)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "F:\eo-reco\.env\Lib\site-packages\transformers\trainer.py", line 1664, in train
    return inner_training_loop(
           ^^^^^^^^^^^^^^^^^^^^
  File "F:\eo-reco\.env\Lib\site-packages\transformers\trainer.py", line 2019, in _inner_training_loop
    self._maybe_log_save_evaluate(tr_loss, model, trial, epoch, ignore_keys_for_eval)
  File "F:\eo-reco\.env\Lib\site-packages\transformers\trainer.py", line 2308, in _maybe_log_save_evaluate
    self._save_checkpoint(model, trial, metrics=metrics)
  File "F:\eo-reco\.env\Lib\site-packages\transformers\trainer.py", line 2462, in _save_checkpoint
    self._push_from_checkpoint(output_dir)
  File "F:\eo-reco\.env\Lib\site-packages\transformers\trainer.py", line 3649, in _push_from_checkpoint
    _, self.push_in_progress = self.repo.push_to_hub(
    ^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: cannot unpack non-iterable NoneType object
```

(Note: line numbers in `run_speech_recognition_ctc.py` will not be accurate, as I've copied it and modified it)

`repo.push_to_hub` can return `None` if the repo is clean, which will cause the issue. However, that might not have happened in my case, since there was no corresponding log message about that (assuming logging would immediately be logged, and not buffered).



### Expected behavior

No exception, maybe just a warning.
cc @Wauplin can we have a consistent return type? That would solve this issue.<|||||>Hmm, what do you mean by _a consistent return type_ ? If nothing is pushed, we can't really return a CommandInProgress object. In general I would prefer not to touch the return type of a method that seems to have been around for 2 years and that might be integrated in a lot of scripts already.

(+ I expect the usage of `Repository` to slowly disappear once we switch to `upload_folder`)<|||||>I mean always a tuple so we don't have to make weird workarounds. But I will do the weird workaround in Transformers to fix this then.