fix(android): null-guard Player in Auth.onListen and getPlayerProfileImage

[AbdullahGhanem]

Problem

PlayersClient.currentPlayer can complete successfully with a null Player (seen in production on a Xiaomi Mi 10 Pro, games_services 5.0.0 and reproducible against current develop). Auth.onListen — which backs GameAuth.player and therefore GameAuth.isSignedIn / GamesServices.isSignedIn — dereferences it immediately:

Fatal Exception: java.lang.NullPointerException
Attempt to invoke interface method 'java.lang.String com.google.android.gms.games.Player.getDisplayName()' on a null object reference
  at com.abedalkareem.games_services.Auth.onListen$lambda$0$0 (Auth.kt:147)
  at com.abedalkareem.games_services.Auth.onListen$lambda$0$1 (Auth.kt:145)
  at com.google.android.gms.tasks.zzm.run

Because this happens on the Android main thread inside a Task callback, a Dart-side try/catch around isSignedIn cannot catch it — the whole app crashes on what should be a harmless "are we signed in?" query.

Fix

Apply the same guard signIn() already has (added in 5.3.0) to the two remaining currentPlayer success listeners:

• onListen: emit FailedToAuthenticate through the event sink (Dart side then resolves isSignedIn to false).
• getPlayerProfileImage: return FailedToGetPlayerProfileImage.

9 lines, Android only, no API change.