That is not true. I (practice) by myself all the time and I can see just fine...but now I have to shave this embarrassing palm hair...
Don has some good advice. (So does Bronson I guess
I practice this way all the time. As far as I'm concerned, training is training. I have discovered many sweet spots in my head without a partner. Also, this kind of training gives you things to "test out" during regular practice and in my opinion teaches you how to answer some of your own questions. We are all supposed to find our own Aikido.
Specifically, I just run through kata over and over. I like to pick techniques that I am having trouble with and try to focus on changing the balance breaks in my head. You will be surprised at how revealing this type of practice can be.
I work offshore on an oil production facility 7 days on and 7 days off, but I practice every day when I am offshore. I have no choice.
Hope these suggestions help.