FSSpec's get_mapper and other functions can produce nameless files for S3 Filesystems

Observed Behavior:

When fsspec's S3FS's get_mapper function is called on an empty directory-like path in an S3 filesystem, it can produce a dictionary-like object that is non-empty, and includes the key '' (i.e. the empty string), and can produce files that are in fact empty directories in its listing:

It appears similar behavior can happen with other functions like ls or walk.

Expected Behavior

1. get_mapper should not produce empty string filenames in directories, as users have to filter these out (they appear not to actually exist, and may correspond to versioning artifacts?).
2. get_mapper should not produce keys that correspond to empty directories.

Both of these behaviors can currently lead to unusual interactions when using get_mapper. For example, when iterating over an FSMap and copying files from the remote system, trying to copy the '' file will produce an error.

Possible Fix

get_mapper for S3FS could include a filter that avoids adding leaf paths that are not files, tested using the isfile predicate.

Start by reproducing the behavior with S3FS's get_mapper on an empty directory-like path, then inspect the related ls and walk behavior and the proposed isfile predicate. Done means listings and mappings omit the empty-string key and entries representing empty directories, with regression coverage for the reported cases.