Use default shutdown reason if the disconnect message is blank.

proxy/src/main/java/com/velocitypowered/proxy/command/builtin/ShutdownCommand.java

@@ -15,8 +15,12 @@ public class ShutdownCommand implements RawCommand {
   @Override
   public void execute(final Invocation invocation) {
     String reason = invocation.arguments();
+    if (reason.isEmpty() || reason.trim().isEmpty()) {
+      server.shutdown(true);
+    } else {
       server.shutdown(true, LegacyComponentSerializer.legacy('&').deserialize(reason));
     }
+  }
 
   @Override
   public boolean hasPermission(final Invocation invocation) {