Disallow null expectations for throws and throwsAsync assertions

[TokenStyle]

Fixes #2410

Second argument called expectation (in the documentation) of the throws and throwsAsync functions, in this change cannot be assigned as null (but can be assigned as undefined).
Also in the error throw message, I changed the text of message, removed null as one of the options for the expectation argument.

[novemberborn]

Thanks for the PR @TokenStyle.

Per #2410, this needs to be behind a flag:

This can partially land behind an experimental flag, to be unflagged when the next major is (pre-)released. The the TypeScript definition can only be updated at that point.

[novemberborn]

Closing due to inactivity. We'd be happy to pick this up again when you have the time to get back to this.