db/view_building_task_mutation_builder: add empty() method

The method allows to check if the builder contains any changes,
so it will allow to skip emitting empty mutation.

db/view/view_building_task_mutation_builder.hh

@@ -52,6 +52,9 @@ public:
     mutation build() {
         return std::move(_m);
     }
+    bool empty() const {
+        return _m.partition().empty();
+    }
 
 private:
     clustering_key get_ck(utils::UUID id);