Cancel event when `CGEventTap` callback returns nullptr

[adamnemecek]

Fixes #664. The fix is taken from https://github.com/tinyx3k/goxkey/blob/38f66edeb65e12a496a50fc700c89492b6215c94/src/platform/macos_ext.rs#L196C5-L213C6

This is absolutely necessary to filter events. Looking around github, returning NULL is a very common use case.

[mattjbray]

See also https://github.com/servo/core-foundation-rs/pull/492

[adamnemecek]

I guess that also works. It is disheartening that that PR been open since 2022.

[jrmuizel]

Is it possible to add a test that shows the problem?

[adamnemecek]

The problem is that currently the api does not allow for consuming events.