I have consistently encountered an error when trying to upload an OCI image resource by digest, ``` 2022-10-19 18:48:50.975 Checking if manifest is already uploaded 2022-10-19 18:48:54.748 Remote image not found, getting its info from local registry. 2022-10-19 18:48:54.752 charmcraft internal error: TypeError("'NoneType' object is not iterable") 2022-10-19 18:48:54.757 Traceback (most recent call last): 2022-10-19 18:48:54.757 File "/snap/charmcraft/1083/lib/charmcraft/main.py", line 180, in main 2022-10-19 18:48:54.757 retcode = dispatcher.run() 2022-10-19 18:48:54.757 File "/snap/charmcraft/1083/lib/craft_cli/dispatcher.py", line 448, in run 2022-10-19 18:48:54.757 return self._loaded_command.run(self._parsed_command_args) 2022-10-19 18:48:54.757 File "/snap/charmcraft/1083/lib/charmcraft/commands/store/__init__.py", line 1778, in run 2022-10-19 18:48:54.757 image_info = dockerd.get_image_info_from_digest(parsed_args.image) 2022-10-19 18:48:54.757 File "/snap/charmcraft/1083/lib/charmcraft/commands/store/registry.py", line 335, in get_image_info_from_digest 2022-10-19 18:48:54.757 if any(digest in repo_digest for repo_digest in image_info["RepoDigests"]): 2022-10-19 18:48:54.757 TypeError: 'NoneType' object is not iterable ``` and it seems that the request to the docker daemon returns a `None` object when the `RepoDigests` key is empty. This is often the case for any images that have been built locally. This PR adds a simple fix and test for that. Then with the recent [change](https://github.com/canonical/charmcraft/pull/871/files) in handling the upload of OCI image resources, I thought it would be helpful to describe exactly what should be included in the ID of an image, originally I was including the "sha26:" portion and that was what was leading me to encountering the bug above. So I've also tweaked the help text output from `charmcraft upload-resource`