Make NullHandler handle 'bubble' argument

[gggeek]

It seems that the code is sending bubble config to Null handlers, but the code does not handle that.
This could be fixed eg. by making NullHandler child of AbstractHandler instead of Handler

See:

        // Handlers using the constructor of AbstractHandler without adding their own arguments
        case 'browser_console':
        case 'test':
        case 'null':
        case 'debug':
            $definition->setArguments([
                $handler['level'],
                $handler['bubble'],
            ]);
            break;

vs

    /**
     * @param string|int $level The minimum logging level at which this handler will be triggered
     */
    public function __construct($level = Logger::DEBUG)
    {
        $this->level = Logger::toMonologLevel($level);
    }

[gggeek]

PS: thinking more about this: what is the use of a null logger if not to be used as 'blackhole'?

Maybe we could leave it as is, but make code more explicit about it never bubbling...

[gggeek]

Ok, there is already a comment in the code that explicitly calls it a 'Blackhole'. I need moar coffee