Fix no stable release sorting bug

[marickvantuil]

This fixes the issue in #562 where if the first fetched package is not stable, then no stable release can be determined because the version comparison will fail with Comparator::greaterThan.

[marickvantuil]

Not sure why the Buddy build is failing, let me know if I can do something to fix it. 👍

[aleho]

This actually fixes the issue on my installation.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b463a21) 99.04% compared to head (8373ebd) 99.04%.

:exclamation: Current head 8373ebd differs from pull request most recent head c5f25d4. Consider uploading reports for the commit c5f25d4 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #617   +/-   ##
=========================================
  Coverage     99.04%   99.04%           
- Complexity     1910     1914    +4     
=========================================
  Files           301      301           
  Lines          6075     6079    +4     
=========================================
+ Hits           6017     6021    +4     
  Misses           58       58           

Impacted Files	Coverage Δ
...ackageSynchronizer/ComposerPackageSynchronizer.php	97.08% <100.00%> (+0.08%)	:arrow_up:

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