timers: improvements to TimersList management

Move all the TimersList instantiation code into the constructor.

Compare values to undefined & null as appropriate instead of
truthy or falsy.

PR-URL: https://github.com/nodejs/node/pull/17429


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>