Support default values for fields of case classes

[mehakun]

Currently, if we perform read of row which contains null, while case class has default value, exception is raised.
I.e.

final case class Obj(userId: Int, userData: String = "data")
// we will receive record (123, null)
cqlt"select user_id, user_data from user_table".as[Obj]

But since cql does not support ifNull in quieries, having its emulation in scala code becomes quite handy and useful