Add JsonNode.isEmpty() as convenience alias

[cowtowncoder]

An often-used idiom for checking for empty Arrays and Object for JsonNode is:

if (arrayNode.size() == 0) { // is empty
}

and due to common use, would make sense to allow use via alias

if (arrayNode.isEmpty()) { ...
}

Semantically it makes sense for containers (Arrays, Objects). For scalars, we could either always return true, or have per-type logic.
I think that former makes more sense at this point even though there may be some confusing in contrast to JsonSerializer.isEmpty() (which does implement much more complex logic).