Use primitive type for returningPlayer

[BlackYps]

returningPlayer is not nullable in the database

[codecov]

Codecov Report

Merging #583 (8b1118b) into develop (b458508) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #583   +/-   ##
==========================================
  Coverage      75.85%   75.85%           
  Complexity      1141     1141           
==========================================
  Files            239      239           
  Lines           3612     3612           
  Branches         231      231           
==========================================
  Hits            2740     2740           
  Misses           776      776           
  Partials          96       96           

Impacted Files	Coverage Δ
...faforever/api/league/domain/LeagueSeasonScore.java	100.00% <ø> (ø)

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