Fix RDB.AggregationCheck when run against an empty group

internal/rdb/rdb.go

@@ -1009,6 +1009,9 @@ func (r *RDB) ListGroups(qname string) ([]string, error) {
 // Returns 1 if an aggregation set was created
 var aggregationCheckCmd = redis.NewScript(`
 local size = redis.call("ZCARD", KEYS[1])
+if size == 0 then
+	return 0
+end
 local maxSize = tonumber(ARGV[1])
 if size >= maxSize then
 	local msgs = redis.call("ZRANGE", KEYS[1], 0, maxSize-1)

internal/rdb/rdb_test.go

@@ -3129,6 +3129,26 @@ func TestAggregationCheck(t *testing.T) {
 		wantAggregationSet []*base.TaskMessage
 		wantGroups         map[string]map[string][]base.Z
 	}{
+		{
+			desc: "with an empty group",
+			groups: map[string]map[string][]base.Z{
+				"default": {
+					"mygroup": {},
+				},
+			},
+			qname:              "default",
+			gname:              "mygroup",
+			gracePeriod:        1 * time.Minute,
+			maxDelay:           10 * time.Minute,
+			maxSize:            5,
+			shouldCreateSet:    false,
+			wantAggregationSet: nil,
+			wantGroups: map[string]map[string][]base.Z{
+				"default": {
+					"mygroup": {},
+				},
+			},
+		},
 		{
 			desc: "with a group size reaching the max size",
 			groups: map[string]map[string][]base.Z{