Render time offsets for cues that begin exactly at 0

[cjcolvar]

In Javascript 0 evaluates to false so a begin time of 0 skips rendering the time span by accident. I switched the guard to check that the typeof the begin time is a number.

0 && true // => 0
20 && true // => true
typeof 0 // => 'number'
typeof 0 === 'number' && true // => true

Resolves #500

[masaball]

👍