It seems my normal dreams are packed with dream characters, but whenever I’m lucid there are very few people around. Kind of annoying, ya know.
One thing you can do before opening a door is to imagine a place where there’s usually a lot of people and they should be there behind that door.
someone else also talked about this in another topic which I can’t remember right now, but you are not the only one that seems to have “empty” lucid dreams (no dream characters). I’m sure someone knows the topic and can put the link here 
I once became lucid inside a restroom (in the dream) and it was packed with women, but they were all faceless…
Woah, that’s pretty creepy. For some reason I’m thinking of Silent Hill.
But on Topic. I’ve had this once or twice, where the switch from normal dreaming to lucidity seems to delete all of the DC’s and I’ve heard a lot of other people talking about empty LD’s so it’s not uncommon and it’s not really something to worry about. I would suggest, as Miles already has, that you try to create people the same way that you would materialise an item in the dream.
One thing that I get more often in LD’s is gobledegook-speaking-idiot-DC’s. Maybe it has something to do with your conscious mind taking over from the subconscious and not being able to create and control as much within the dream as your subconscious could.