[BUGFIX] Ensure valid arguments for array_chunk

Fixes #637
Well, only if $count is zero. But do we need to take care of that? We also don't check it's not negative etc. :-)
Imho $count does not change by the application but by the integrator using the VH - while the array-size can of course be zero just by application-logic.

So you'd favor checking $count as well? What do we do for a count smaller than 1? Return an empty array?