Allow creating collection element with all nullable types if at least one property value is available

[denis-zhdanov]

Problem

This is a variation of the #14 - suppose that we have a use-case like below:

data class First(val second: Collection<Second>)

data class Second(val i: Int?, val third: Set<Third>?)

data class Third(val j: Int)

and the following data:

second[0].i = 1

Here all Second properties are nullable, but i value is defined, hence, Second.third property should be tolerated as null