Fix the inability to override commands (Backport of https://github.com/CloudburstMC/Nukkit/pull/2196)

[Yexeed]

This PR backports from upstream pull CloudburstMC/Nukkit#2196

tl;dr fixes command overriding which is not possible without this changes
sample code:

public static boolean unregisterCommand(Command command){
    command.setLabel(command.getName() + "__unregistered");
    return command.unregister(Server.getInstance().getCommandMap());
}

[lt-name]

I think different judgments need to be made based on usage.
It should not be null when registering a command

[Yexeed]

I think different judgments need to be made based on usage.
It should not be null when registering a command

What? Just try to override a command without this PR, it wont work because this if-block was broken 8 years ago, mismatching "this.commandMap" with "commandMap". With this PR it's once again possible to override a command. However, why should we make some judjements about something THAT simple? Command SHOULD allow changes from foreign command map if:

• the command itself doesnt have a valid command map attached to it (this.commandMap == null)
  or
• the command has command map equal to passed command map (this.commandMap == commandMap)
  Its that simple

[Yexeed]

Also, registering a command (not overriding it) works fine

[lt-name]

public boolean register(CommandMap commandMap) {
    if (this.allowChangesFrom(commandMap)) {
        this.commandMap = commandMap;
        return true;
    }
    return false;
}

I mean register(null) will return true
After this pr merge, although unregister is fixed, register will be broken

[Yexeed]

Null safety check should fix the problem?