refactor: optimise "empty" case in report set constructors

report/id_list.go

@@ -7,11 +7,17 @@ var emptyIDList = IDList(MakeStringSet())
 
 // MakeIDList makes a new IDList.
 func MakeIDList(ids ...string) IDList {
+	if len(ids) == 0 {
+		return emptyIDList
+	}
 	return IDList(MakeStringSet(ids...))
 }
 
 // Add is the only correct way to add ids to an IDList.
 func (a IDList) Add(ids ...string) IDList {
+	if len(ids) == 0 {
+		return a
+	}
 	return IDList(StringSet(a).Add(ids...))
 }
 

report/node_set.go

@@ -27,6 +27,9 @@ func MakeNodeSet(nodes ...Node) NodeSet {
 // Add adds the nodes to the NodeSet. Add is the only valid way to grow a
 // NodeSet. Add returns the NodeSet to enable chaining.
 func (n NodeSet) Add(nodes ...Node) NodeSet {
+	if len(nodes) == 0 {
+		return n
+	}
 	result := n.psMap
 	if result == nil {
 		result = ps.NewMap()