Small PHP adjustments in Chisel methods

[JakubSzajna]

Hey :)

As I was recently working with Chisel on a bigger project I've noticed two small adjustments I would add to the code.

The "default value" adjustments could be simplified to return $value ?? $default_value; but our x5view does not use php7 by default yet and this would fail on the preview.

[JakubSzajna]

I've also changed JS className function definition to be consistent with the PHP one (thanks to @jakub300 suggestion), but I did not test it.

[luboskmetko]

thanks @JakubSzajna

@jakub300 @marcinkrzeminski could you please review this so we can merge it?