Null validation produces a false negative on nullable collections that contain null

[robpridham]

If you run the null validation in containsIllegalNullValues on a nullable list, e.g.:

val nullableListWithNull = listOf<String?>(null)

then it will return false, because on iterating through the list, element is null. This may seem like an odd scenario but we experience it when using Gson with a custom deserialiser that ignores certain elements, for which it returns null.

I did look at whether it was possible to improve on this, e.g. examine the return type of the collection's iterator to see if isMarkedNullable would be true, but I couldn't find a working solution. I think this a product of how Kotlin does generics, e.g. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect/-k-type/is-marked-nullable.html