SIP public review: explicit Nulls

[eyalroth]

@sjrd

Let us please stay focused on the proposal at hand in this thread, and stay out of any discussion of whether or not nulls and exceptions are appropriate in tests and/or production code.

That’s not entirely what the comments were about. They were also about two other on-topic issues:

1. Whether a new special syntax is worth the cost (as @jducoeur mentioned).
2. Whether the SIP is compatible with Mockito, which is a commonly used library.