I've died in my dreams more times than I can count. I usually don't experience all black nothingness though. Generally, it's like, "fuck, I'm dead," then I'll either wake up, or I'll segue into another dream. The worst is when I die in my dreams in a completely "normal"/"realistic" way, like drowning, hit by a car, shot, stabbed, etc, etc. It's one thing to die in a dream because you were eaten by a dragon, but another to die in a dream where you were robbed at gunpoint. That shit sucks.
I always assume it's normal to die in dreams, and I don't think I ever heard it was impossible to die in a dream. Even so, it's happened so many times for me that I'd have ignored the people saying you can't anyway XD