。
The fact that nobody died was a small mercy amid the disaster.
Literal
Anyone didn't-die [nominalizer-のは] in-misfortune [gen-の] fortune [polite-でした].
誰も + [negative] 'no one,' a universal quantifier requiring negative agreement. ~のは nominalizes the preceding clause as the topic of evaluation. 不幸中の幸い literally 'fortune in the midst of misfortune' is the stock idiom for 'a small mercy within a disaster / a silver lining.' A canonical Japanese expression for looking at the bright side of something terrible.