Empty JsonObject and JsonArray constants

[MukjepScarlet]

Feature description

Creating a new complete instance every time you need an empty JsonObject or JsonArray is a waste of memory. (creating LinkedTreeMap and ArrayList with default size)

public static final JsonObject EMPTY
public static final JsonArray EMPTY

They are immutable and always constant. So we don't even need to include any Map or List in them.

[eamonnmcmanus]

I'm not sure about this. Do you see a lot of use for them? I'm a little concerned that JsonObject and JsonArray have always been mutable, and if we suddenly introduce immutable variants they may interact poorly with existing code.