Erm this might be a bit silly, but would be agender be like when you don’t really feel anything when being called a boy or a girl?

That can definitely be a part of it, someone who is agender may not feel a connection to being called either a boy or a girl. It doesn't necessarily mean so, as even someone cisgender could potentially not feel a reaction to either of those, but it can also be something an agender person would experience. It's certainly not silly to ask, gender is a very complicated, internal feeling that can be hard to fully grasp, and these sorts of 'clues' can often help people better understand what they're feeling inside.