fix: Null check when null is added to team

src/main/java/cz/muni/fi/pa165/icehockeymanager/services/TeamServiceImpl.java

@@ -35,6 +35,9 @@ public class TeamServiceImpl implements TeamService {
 
     @Override
     public void addPlayerToTeam(Team team, Player player) {
+        if (player == null){
+            throw new NullPointerException("Player is null");
+        }
         team.addPlayer(player);
         teamDao.update(team);
     }