fix(lib): a null clock must have tv_nsec be 0 as well

[amaanq]

Closes #3341

In a WASM environment, it seems like calling clock_gettime with CLOCK_MONOTONIC gives us a really small time at first; it could be related to when the program starts, I'm not sure. The problem is our check for clock_is_null only checks if tv_sec is 0, and when we first set end_clock, it is set to a value where tv_sec is 0, and tv_nsec is some large number, around 0.05-0.1s in my testing. So we should also check for tv_nsec being non-zero before assuming a clock is null/none.