I think +=0 is accurate, since the idea is that you're performing a mathematic operation on the node's reputation, which is an integer. If you make the null vote say "null", it doesn't follow the pattern set by the ++ and -- options. Instead, I propose that we change the name of the null vote option to the "increment by zero" option, which is much more descriptive and only 6 times the number of syllables.
Well in a way that is my point, when you vote +=0 it appears as though you are performing an operation on the node's reputation, but it doesn't actually do anything (I wonder what the code for this bit looks like).
And again I say, Null is not equal to zero. But there I go with the pedantry again.