Add @NotNull to Key in org.bukkit.sound (#5241)

Fix compiling

Spigot-API-Patches/Adventure.patch

@@ -594,6 +594,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      }
 +
 +    // Paper start
++    @NotNull
 +    @Override
 +    public net.kyori.adventure.key.@org.checkerframework.checker.nullness.qual.NonNull Key key() {
 +        return this.key;