Fix sorting multiple fields when the first is equal between documents

src/sort.js

@@ -45,7 +45,7 @@ module.exports = (_next, spec) => {
         for (var [path_pieces, order] of sorts) {
             const result = compare(a, b, path_pieces, order);
 
-            if (result !== null) { return result; }
+            if (result > 0 || result < 0) { return result; }
         }
 
         return -order;