`metadata_update` fails when no README.md exists

Describe the bug

metadata_update cannot be called on a repo with no README.md file and throws the following exception:

EntryNotFoundError: 404 Client Error: Entry Not Found for url: https://huggingface.co/datasets/cakiki/abc/resolve/main/README.md. (Request ID: SWDb5XnCP0Ofk0koTZuva)

Reproduction

Create a dataset without choosing a license and call update_metadata

e.g.: metadata_update("cakiki/abc", {"license": "cc-by-sa-4.0"}, repo_type="dataset", overwrite=True)

Logs

---------------------------------------------------------------------------
EntryNotFoundError                        Traceback (most recent call last)
/tmp/ipykernel_117739/1604102178.py in <module>
----> 1 metadata_update("cakiki/abc", {"license": "cc-by-sa-4.0"}, 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)
    175         filename=REPOCARD_NAME,
    176         repo_type=repo_type,
--> 177         use_auth_token=token,
    178     )
    179     # work on a copy of the upstream file, to not mess up the cache

/media/ssd/BIGSCIENCE/env/lib/python3.7/site-packages/huggingface_hub/file_download.py in hf_hub_download(repo_id, filename, subfolder, repo_type, revision, library_name, library_version, cache_dir, user_agent, force_download, force_filename, proxies, etag_timeout, resume_download, use_auth_token, local_files_only, legacy_cache_layout)
   1018                 timeout=etag_timeout,
   1019             )
-> 1020             _raise_for_status(r)
   1021             commit_hash = r.headers[HUGGINGFACE_HEADER_X_REPO_COMMIT]
   1022             if commit_hash is None:

/media/ssd/BIGSCIENCE/env/lib/python3.7/site-packages/huggingface_hub/utils/_errors.py in _raise_for_status(request)
     70         elif error_code == "EntryNotFound":
     71             raise EntryNotFoundError(
---> 72                 f"404 Client Error: Entry Not Found for url: {request.url}. (Request"
     73                 f" ID: {request_id})"
     74             )

EntryNotFoundError: 404 Client Error: Entry Not Found for url: https://huggingface.co/datasets/cakiki/abc/resolve/main/README.md. (Request ID: SWDb5XnCP0Ofk0koTZuva)

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

[nateraw]

What would you expect here, @cakiki ? I assume if no readme exists, we should just create a new card with the specified metadata and push that up with empty text content after it. WDYT?

[osanseviero]

cc @nateraw