Fix Void flatbuffers serialization

Before this fix, the encoding of Void happens to 'work' in that this
implementation can both read and write it, but it is not a flatbuffers
message since it contains a invalid offset (the implementation 'works'
because it does not dereference that offset).

flow/flow.h

@@ -123,7 +123,9 @@ class Void {
 public:
 	constexpr static FileIdentifier file_identifier = 2010442;
 	template <class Ar>
-	void serialize(Ar&) {}
+	void serialize(Ar& ar) {
+		serializer(ar);
+	}
 };
 
 class Never {};