Changes to realizability once we have a type system that tracks nullability

[abeln]

Consider what, if any, changes are required to CheckRealizable in light of this comment

// check fields only under strict mode for now.
 // Reason: An embedded field could well be nullable, which means it
// should not be part of a path and need not be checked; but we cannot recognize
// this situation until we have a typesystem that tracks nullability.

https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/CheckRealizable.scala#L178