[ticket/12536] Get Versions Should Not Require Both Stable and Unstable

https://tracker.phpbb.com/browse/PHPBB3-12536

PHPBB3-12536
[ticket/12536] Return empty array if stability unavailable

PHPBB3-12536
[ticket/12536] Update doc block

PHPBB3-12536
[ticket/12536] Use stable values when unstable are unavailable

PHPBB3-12536
[ticket/12536] Add test cases with empty versions list

PHPBB3-12536
[ticket/12536] Restore missing @throws

PHPBB3-12536
Merge pull request #2453 from Nicofuma/ticket/12536

[ticket/12536] Get Versions Should Not Require Both Stable and Unstable

* Nicofuma/ticket/12536:
  [ticket/12536] Restore missing @throws
  [ticket/12536] Add test cases with empty versions list
  [ticket/12536] Use stable values when unstable are unavailable
  [ticket/12536] Update doc block
  [ticket/12536] Return empty array if stability unavailable
  [ticket/12536] Get Versions Should Not Require Both Stable and Unstable