Fix smb:/ handling

Summary:
smb:// is a URL with no host and no path
smb:/// is a URL with no host, but a valid path

The current code special cases smb:/ which isn't a real thing.

This results in smb:// falling through checkURL() and being incorrectly
modified into smb:/// creating a URL with a host, something different.

This code makes any smb:/ get converted into smb:// at the right place.
Then the special case for having no host or path are handled properly.

Test Plan: https://paste.kde.org/pfpwrlxva

Reviewers: apol, madcatx, elvisangelaccio, ngraham

Reviewed By: elvisangelaccio, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D8141