GH-138 Fix leaderboards null user.

[ChudziudgiToJa]

[27-11-2023 18:05:52] JDA             ERROR  One of the EventListeners had an uncaught exception
java.lang.NullPointerException: Cannot invoke "net.dv8tion.jda.api.entities.Member.getEffectiveName()" because the return value of "net.dv8tion.jda.api.entities.Guild.getMemberById(long)" is null
        at com.eternalcode.discordapp.leveling.leaderboard.LeaderboardCommand.execute(LeaderboardCommand.java:48)
        at com.jagrosh.jdautilities.command.SlashCommand.run(SlashCommand.java:322)
        at com.jagrosh.jdautilities.command.impl.CommandClientImpl.onSlashCommand(CommandClientImpl.java:936)
        at com.jagrosh.jdautilities.command.impl.CommandClientImpl.onEvent(CommandClientImpl.java:626)
        at net.dv8tion.jda.api.hooks.InterfacedEventManager.handle(InterfacedEventManager.java:96)
        at net.dv8tion.jda.internal.hooks.EventManagerProxy.handleInternally(EventManagerProxy.java:88)
        at net.dv8tion.jda.internal.hooks.EventManagerProxy.handle(EventManagerProxy.java:70)
        at net.dv8tion.jda.internal.JDAImpl.handleEvent(JDAImpl.java:177)
        at net.dv8tion.jda.internal.handle.InteractionCreateHandler.handleCommand(InteractionCreateHandler.java:112)
        at net.dv8tion.jda.internal.handle.InteractionCreateHandler.handleInternally(InteractionCreateHandler.java:83)
        at net.dv8tion.jda.internal.handle.SocketHandler.handle(SocketHandler.java:39)
        at net.dv8tion.jda.internal.requests.WebSocketClient.onDispatch(WebSocketClient.java:1015)
        at net.dv8tion.jda.internal.requests.WebSocketClient.onEvent(WebSocketClient.java:901)
        at net.dv8tion.jda.internal.requests.WebSocketClient.handleEvent(WebSocketClient.java:879)
        at net.dv8tion.jda.internal.requests.WebSocketClient.onBinaryMessage(WebSocketClient.java:1054)
        at com.neovisionaries.ws.client.ListenerManager.callOnBinaryMessage(ListenerManager.java:385)
        at com.neovisionaries.ws.client.ReadingThread.callOnBinaryMessage(ReadingThread.java:276)
        at com.neovisionaries.ws.client.ReadingThread.handleBinaryFrame(ReadingThread.java:996)
        at com.neovisionaries.ws.client.ReadingThread.handleFrame(ReadingThread.java:755)
        at com.neovisionaries.ws.client.ReadingThread.main(ReadingThread.java:108)
        at com.neovisionaries.ws.client.ReadingThread.runMain(ReadingThread.java:64)
        at com.neovisionaries.ws.client.WebSocketThread.run(WebSocketThread.java:45)

[vLuckyyy]

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