[fix] Allow kv_clear to work on tag-only entries with no field data

[dodatboii]

Previously, calling kv_clear on a key that was put via async_kv_put with only a tag (e.g. {"status": "finished"}) and no fields would raise:

RuntimeError: Fail to clear_data for key-value based backends due to lack of field_names in BatchMeta

This happened because async_clear_samples always called storage_manager.clear_data, which requires field_names to generate storage keys. For tag-only entries, nothing was ever written to the storage backend, so there is nothing to clear there.

Fix: skip storage_manager.clear_data when metadata.field_names is empty, and only clear the controller-side metadata.

[ascend-robot]

CLA Signature Pass

dodatboii, thanks for your pull request. All authors of the commits have signed the CLA. 👍

[0oshowero0]

I believe we should not simply skip storage clear