fix issue with FileSystem::glob handling of non-existent files

[a10y]

Summary

Closes: #6599

When FileSystem::glob() is called without any glob patterns, it returns a file stream with either a single element or no elements, depending on if the file exists in the FS.

API Changes

None

Testing

3 unit tests:

1. globbing single file
2. Globbing non-existent file
3. Globbing file where multiple have same prefix only returns stream for exact-match

[a10y]

There's something weird here, since ObjectStore::list("x") doesn't return "x" if "x" is a file path. This probably needs to use head_object instead, and plumb that through FileSystem