search: defensive nil-check for and-results

Created by: rvantonder

This is a defensive change to check whether result is nil before it is potentially accessed like result.limitHit in this function. I believe it is hard, but not impossible, to induce the value of nil for result here, but haven't been able to trigger it (the value depends on evaluateLeaf, our base search logic, and not something dependent on operators). The ideal solution would be to ensure that result is non-nil (pagination ensures this, for example), but that change would be too involved right now.

This change is checked against search integration tests, and again, it's a defensive change so shouldn't have any meaningful impact to logic.