O.k., I see now that you actually do have a CVX-representable formula for your desired expression, although you need to divide by log(2).
As for t(n)=0, t(n) is input data. As I wrote earlier, the (original) expression evaluates to zero when t(n) = 0. So treat it accordingly in your program, i.e., use zero rather than the rel_entr formula when t(n) = 0. Such conditional is allowed because it is conditional on input data, not on a CVX variable or expression.
Note: I should have recognized that your formula is the already known formula at Writing x*log(1+x/y) , with both sides divided by x, which works as long as x is a non-zero constant.