events: stricter prop & variable checks for perf

Replace truthy/falsey checks of _events and _events[type] with
comparisons to undefined for better performance:

events/ee-add-remove.js n=250000    5.30 %    *** 4.260028e-07
events/ee-emit.js n=2000000         4.18 %    *** 1.026649e-05

This has a knock-on effect on modules that use lots of events, e.g.:

http2/headers.js nheaders=0 n=1000  2.60 %    *** 0.000298338

PR-URL: https://github.com/nodejs/node/pull/16212


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>