These reservations that Yoda may have about Luke, and the relatively brief time that he had to train him are the likely reasons why Yoda never trained him to defend himself against Force lightning.
I still don't see what defending
yourself against force lightning has to do with the dark side. I didn't get the impression that Mace using his saber or Yoda using his hand to repel the force lightning caused them to tap into dark energies. How would telling Luke about it and showing him how to defend against it lead him to the dark side?
Luke: Is the dark side stronger?
Yoda: No, no, no. Quicker, easier, more seductive.
Luke: But how am I to know the good side from the bad?
Yoda: You will know... when you are calm, at peace, passive. A Jedi uses the Force for knowledge and defense, NEVER for attack.
Yoda is very clear about using powers for defense and distinguishing between using the force for good or evil. Given his dialogue, he could very easily follow the above with "by the way, the Emperor seems old and weak but he can shoot lightning out his fingertips." Whether he did so or not, I don't see any reason to purposely hide it from Luke or any way that drives Luke to the dark.
Now, I can totally see where Yoda just didn't get to it because Luke left too soon or he prioritized other aspects of the training over the force lightning defense or Luke just isn't strong enough in the force yet to handle it, so he doesn't even go there. In fact, the more I think about it I wonder if Yoda even thought Luke would be confronting the Emperor. He seems to be preparing him to face Vader and maybe he figured if Luke survived that, then he and Obi could further instruct Luke on how to handle the Emperor.