refactor: assert value for RxJS v7 compatibility

[MichaelJamesParsons]

In RxJS v7, the type of Subject.next() will change from next(value?: T) to next(value: T) (change). No runtime behavior is affected, but this change will break users who build from source. This PR corrects types to ensure projects build successfully.

Changes

• asserts T|undefined|null values to T where appropriate.
• refactor instances of Subject<any> to Subject<void> so the next() method can be called without passing an argument.

[jelbourn]

@mmalerba PTAL

[jelbourn]

All the void changes look good, Miles should weigh in on the virtual scroll change

[crisbeto]

LGTM

[mmalerba]

LGTM

[jelbourn]

@MichaelJamesParsons can you update the API gold for snackbar?

yarn bazel run //tools/public_api_guard:material/snack-bar.d.ts_api.accept

[crisbeto]

The shorthand for updating the golden is yarn approve-api material/snack-bar.

[MichaelJamesParsons]

Done

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