Currently, I'm making a game with a friend. I'm the only programmer and the assets are going to take quite a while until they are ready.

I've already made enough functionality, testing it using cubes and other basic shapes to represent the scene (items, players, enemies, etc) and I'm wondering how much can I do before the assets are ready.

Can I make the whole game without having any assets at all?

I'm not very experienced, so I'm wondering if there are some things that should wait to be implemented when I have the assets. I don't want to make something and later realize that I have to make it from scratch.

All the things that require some sort of graphics have been constructed as empty object prefabs ready to construct other prefab variants with different graphics.

Can I make the whole game without having any assets at all?

Not at all, usually many problems arise in pipeline when incorporating real deal assets. Unless you have prototyped all asset types, consider being in pre-alfa stage of your game.

True, a cube is a box, so it has a simple box collider, while a real asset may be more complex and require mesh collision. This is quite more complex and costly so your game might run slower than expected. Animations could not play well with your animation system and shaders may impact the game also. It is best to have some kind of development assets when you start to work on a game's code base