Location.canonical_path: some fixes, see #8446

[ThomasWaldmann]

1st commit: just fixing the protocol part:

% borg repo-info
Location: rclone://None/./pcloud:justtesting

2nd commit: do not display None, but rather empty host:

% borg repo-info    
Location: rclone:///./pcloud:justtesting

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.64%. Comparing base (de439ee) to head (136d3dc). Report is 11 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8447 +/- ## ======================================= Coverage 81.64% 81.64% ======================================= Files 71 71 Lines 12906 12906 Branches 2337 2337 ======================================= Hits 10537 10537 Misses 1707 1707 Partials 662 662 ```

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