Explain in comment why [[nil]] is considered empty result

src/metabase/pulse.clj

@@ -70,6 +70,7 @@
   [card]
   (let [result (:result card)]
     (or (zero? (-> result :row_count))
+        ;; Many aggregations result in [[nil]] if there are no rows to aggregate after filters
         (= [[nil]]
            (-> result :data :rows)))))
 

src/metabase/pulse/render.clj

@@ -391,6 +391,7 @@
       (or (= aggregation :rows)
           (contains? #{:pin_map :state :country} (:display card))) nil
       (or (zero? row-count)
+          ;; Many aggregations result in [[nil]] if there are no rows to aggregate after filters
           (= [[nil]] (-> data :rows)))                             :empty
       (and (= col-count 1)
            (= row-count 1))                                        :scalar