fix: return null instead of crashing when no solution is found

[sven-strothoff]

When no solution can be found cubejs crashes while trying to access null.

This happens when maxDepth is set too low for the current scramble. To reproduce this bug just set it to 1 (see also the added test).

I usually don't use CoffeScript, so I hope the syntax is ok. Otherwise, feel free to edit it. :)