Multistore: If no files are uploaded to published dataset, export fails.

[kcondon]

Discovered while testing export/import where test case involved a published dataset with no files. If using a local file store, a published dataset with no files fails to export, returns a json file with error. If add files then publish, works as expected.

[landreev]

My guess is, the new ("multistore") code only creates the dataset directory on a file store when it needs to save a DataFile; for dataset-level files, the code assumes it already exists, and fails if it does not.

[landreev]

OK, on closer inspection, this was introduced NOT in the original multistore PR, but in #6670, the followup PR that specifically addressed some issues with export in the first multistore PR (#6488)