This variable is only a ‘handle’, a ‘pointer’, a reference to a fully independent scheduled timer. Setting the variable to null has no effect on an existing timer. However, the handle (when still valid!) allows you to communicate with the timer, by sending cancel, reschedule etc.
Many rule authors use setting handle to null just as a convenient indicator that some previously defined timer is now finished with. That’s a really useful trick, but has nothing to do with documented timer syntax.