fix for sum(null)

src/query/plan/operator.cpp

@@ -2599,8 +2599,8 @@ namespace {
 TypedValue DefaultAggregationOpValue(const Aggregate::Element &element, utils::MemoryResource *memory) {
   switch (element.op) {
     case Aggregation::Op::COUNT:
-      return TypedValue(0, memory);
     case Aggregation::Op::SUM:
+      return TypedValue(0, memory);
     case Aggregation::Op::MIN:
     case Aggregation::Op::MAX:
     case Aggregation::Op::AVG:

tests/unit/query_plan_accumulate_aggregate.cpp

@@ -1,4 +1,4 @@
-// Copyright 2021 Memgraph Ltd.
+// Copyright 2022 Memgraph Ltd.
 //
 // Use of this software is governed by the Business Source License
 // included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
@@ -260,7 +260,7 @@ TEST_F(QueryPlanAggregateOps, WithoutDataWithoutGroupBy) {
   // max
   EXPECT_TRUE(results[0][3].IsNull());
   // sum
-  EXPECT_TRUE(results[0][4].IsNull());
+  EXPECT_EQ(results[0][4].ValueInt(), 0);
   // avg
   EXPECT_TRUE(results[0][5].IsNull());
   // collect list