Skip calling ticks when the sleep queue is empty

[niaow]

This was a micro-optimization I worked on when messing around with #513. I later put it in #606.

Basically:

1. When running scheduler-heavy code (in the original case a goroutine/channel benchmark), the scheduler gets bottlenecked by expensive calls to check the time on the system. This patch allows us to skip the check and get a massive improvement in performance.
2. When a piece of code does not care about timing, sleepQueue != nil gets constant-folded to false and all of the timing code gets optimized away.

[aykevl]

Great find!