In order to temporarily disable event listening for a specific handle,
it may be convenient to reregister it with an empty interest.

This avoids to temporarily deregister the handle before registering it
again (which, by the way, is not supported on Windows[1]).

Therefore, remove the check that prohibited registration with empty
interest.

Note that ERROR and HUP may still be triggered in that case.

[1]: tokio-rs#633

Assert that poll() does not generate a readable event when registered
with empty interest, but that it generates the pending readable event as
soon as we reregister with a readable interest.