folds/quotient-sloppy divides by zero

[jstokes]

The documentation for folds/quotient-zero states that

Like quotient, but considers 0/0 = 0.

However, the current implementation only checks that the first event's metric value is zero, and if so short circuits the fold-all over the rest of the metrics. This doesn't actually protect against the divide by zero case when we have a non-zero metric followed by any number of zero metric events.

[jamtur01]

Thanks! Merged and released.