I don't think Emerson really needs a nickname, but if I had to shorten it I'd probably go with Emer. It seems logical and not girlish like Em. Emers seems a bit cutesy, like Jakers for Jake or something. I know it's actually in the name-- EMERSon -- but when I see it like that I just see Emer with n S. Emery would work too, and I like it, but I'd probably go with Emer.