Constructive critique is meant to be just that. It is criticism that helps one improve, and that means it doesn't necessarily have to be "nice." Someone giving constructive critique is in no way obligated to tell you what they like about your piece. As long as their criticism gives you something to work off, something that will help you improve, then it is absolutely legitimate.
I'm not necessarily saying this is the only right way to critique. I know there are beginners out there who would love this kind of critique once they are more comfortable with their work. Of course compliments are nice to get. And it's fine to give them.
But constructive critique does not always need to come with compliments. Someone who doesn't tell you anything about what they liked in your piece is not being mean.