get-url, import-url: handle empty directory imports

[isidentical]

When getting/importing an empty directory, we don't take any actions in the workspace since we don't create the parent directories unless there are files.

dvc import-url some://empty/dir/
dvc get-url some://empty/dir/

[isidentical]

Awaiting fixes on the upstream implementations (adlfs, gcsfs) before we can act.

[jorgeorpinel]

When getting/importing an empty directory, we don't take any actions in the workspace

Actually it currently fails with some repetitive and contradicting messaging 🙂

$  dvc import-url ../ext/empty
Importing '../ext/empty' -> 'empty'
WARNING: '../ext/empty' is empty.
WARNING: '../ext/empty' is empty.
ERROR: failed to import ../ext/empty. You could also try downloading it manually,
and adding it with `dvc add`. - output 'empty' does not exist

Contradiction: "is empty" vs. "does not exist"

DVC 2.0.5 (pip)

[isidentical]

Awaiting: fsspec/filesystem_spec#562 (comment)