fix(useStorageValue): properly handle storage-event removal

[ranneyd]

What is the current behavior, and the steps to reproduce the issue?

When you have two tabs, and one of them removes an item in local storage, the second one will ignore that update. Setting the value works, just not removing it.

What is the expected behavior?

Any operation to a local storage key in one tab is synchronized with the other tab.

How does this PR fix the problem?

This synchronization happens via the window's storage event. When a value is removed, newValue is set to null. The handler for that event ignores events where the value is null. This simply removes that check.

I don't think this behavior is intentional. That check was added in #960, which was a large refactor. The check wasn't there before, so I think this fell through the cracks.

Checklist

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