`metadata_update` fails when README.md has no text content (but it has metadata)

Describe the bug

metadata_update fails when there is no newline after the YAML header.

I had a README.md file with the following 3 lines:

---
license: cc-by-sa-4.0
---

calling metadata_update("cakiki/abc", {"tag": "test"}, repo_type="dataset", overwrite=True) throws TypeError: argument of type 'NoneType' is not iterable.

I had to add a new line, i.e. content to the README for this to work.

Reproduction

No response

Logs

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_117739/230484724.py in <module>
----> 1 metadata_update("cakiki/abc", {"tag": "test"}, repo_type="dataset", overwrite=True)

/media/ssd/BIGSCIENCE/env/lib/python3.7/site-packages/huggingface_hub/repocard.py in metadata_update(repo_id, metadata, repo_type, overwrite, token, commit_message, commit_description, revision, create_pr)
    199             # update all fields except model index
    200             else:
--> 201                 if key in existing_metadata and not overwrite:
    202                     if existing_metadata[key] != metadata[key]:
    203                         raise ValueError(

TypeError: argument of type 'NoneType' is not iterable

System Info

- `transformers` version: 4.21.1
- Platform: Linux-5.15.0-46-generic-x86_64-with-glibc2.29
- Python version: 3.8.10
- Huggingface_hub version: 0.9.0.dev0
- PyTorch version (GPU?): 1.11.0+cu102 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed

[Wauplin]

Oops, closed it automatically with the PR. Please open it again if necessary @cakiki

[Wauplin]

Seems that @nateraw already fixed it without knowing 😄 I created a PR to add a regression test but that’s it. Please let me know if I missed something.