Support "?:" in object constructor

The current implementation do not support separation by "?:" between key, value pair.
Implementation details:
If the right operand evaluates to the empty sequence, the value of the pair is the atomic value null (of type js:null). However, if the colon is preceded with a question mark, the pair is omitted instead.
If the right operand evaluates to a sequence of more than one item, the value of the pair is an array containing copies of all items in this sequence, in the same order.
[1] http://jsoniq.org/docs/JSONiqExtensionToXQuery/html-single/index.html#idm139680686311712