Deny permission to use a command if no permission is set

Having no permission is almost always a bug. We already have behaviour elsewhere in the core that assumes undefined permission = permission denied.

This behaviour might confuse some people, but I think it's much less dangerous than accidentally allowing everyone to use your command.

src/command/Command.php

@@ -113,7 +113,7 @@ abstract class Command{
 	public function testPermissionSilent(CommandSender $target, ?string $permission = null) : bool{
 		$permission ??= $this->permission;
 		if($permission === null || $permission === ""){
-			return true;
+			return false;
 		}
 
 		foreach(explode(";", $permission) as $p){