Require non-null reason

[WouterGritter]

A plugin providing a null reason to Player#disconnect will get a vague NullPointerException pointing towards Velocity's/Adventure's internals, even though the problem is a proxy plugin providing a null reason.

This PR makes this non-null contract explicit and throws with a clear NPE.

Exception:

[07:17:53 INFO]: [connected player] xDevanger (/REDACTED): kicked from server lobby: ᴍᴄꜰᴜɴɴʏ.ᴘʟ » Serwer ʟᴏʙʙʏ jest w trakcie restartu...
[07:17:53 INFO]: [server connection] xDevanger -> lobby has disconnected
[07:17:53 WARN] [io.netty.util.concurrent.AbstractEventExecutor]: A task raised an exception. Task: com.velocitypowered.proxy.connection.client.ConnectedPlayer$$Lambda/0x0000000069e10a30@61031780
java.lang.NullPointerException: like
        at java.base/java.util.Objects.requireNonNull(Objects.java:246) ~[?:?]
        at net.kyori.adventure.text.Component.append(Component.java:2112) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at net.kyori.adventure.text.ScopedComponent.append(ScopedComponent.java:99) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at net.kyori.adventure.text.Component.append(Component.java:2101) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at net.kyori.adventure.text.ScopedComponent.append(ScopedComponent.java:93) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at com.velocitypowered.proxy.connection.client.ConnectedPlayer.disconnect0(ConnectedPlayer.java:890) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at com.velocitypowered.proxy.connection.client.ConnectedPlayer.lambda$disconnect$3(ConnectedPlayer.java:872) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:148) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:141) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:535) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.channel.SingleThreadIoEventLoop.run(SingleThreadIoEventLoop.java:201) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:1195) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
        at com.velocitypowered.proxy.util.concurrent.VelocityNettyThreadFactory$1.run(VelocityNettyThreadFactory.java:46) ~[velocity-ctd-fatjar-3.5.0-SNAPSHOT-301.jar:3.5.0-SNAPSHOT-git-2878e6c7-b301]
[07:17:53 INFO]: [server connection] xDevanger -> limbo has connected