Fix PlackettLuce handling of ties

[smoogipoo]

Upstream went through its own bugfix in vivekjoshy/openskill.py#176, resulting in the test data being updated, which shows a further likely transcription issue in the code here.

Compare: https://github.com/vivekjoshy/openskill.py/blob/c9d3210ed068570cfa11a3f56586255c93d1a4c8/openskill/models/weng_lin/plackett_luce.py#L825-L828

[smoogipoo]

Bundled another fix here for an exception when all scores are tied, because Average() throws if there are no elements.

Sequence contains no elements
   at System.Linq.ThrowHelper.ThrowNoElementsException()
   at System.Linq.Enumerable.Average[TSource,TAccumulator,TResult](IEnumerable`1 source)
   at OpenSkillSharp.OpenSkillModelBase.<>c__DisplayClass45_0.<CalculateMarginAdjustedMu>b__1(ITeamRating qTeam, Int32 qTeamIndex) in /home/smgi/Repos/OpenSkillSharp/OpenSkillSharp/OpenSkillModelBase.cs:line 276

Compare: https://github.com/vivekjoshy/openskill.py/blob/c9d3210ed068570cfa11a3f56586255c93d1a4c8/openskill/models/weng_lin/plackett_luce.py#L790-L815 (notice if comparison_count else margin_adjustment)

[myssto]

Thanks for this! Looks good as far as I can see but I will take a closer look soon and get this merged.