Don't try to array_combine two empty arrays

This fixes the case where there are no consumers attached to the analytics instance. Basically, `array_combine` requires that both arrays have a value, otherwise it throws an error. We shouldn't be calling `array_combine` with two empty arrays in the first place--the result will always be an empty array in that case--so this is a minor optimization, but it also prevents unit tests from failing (since they never have consumers attached).

ping @PascalZajac 
If one of the two array is empty (number of elements mismatch), PHP will still throw a warning. Correct me if I'm wrong, but I think this logic doesn't solve number 2:

1) Both are empty
2) One is empty
3) None are empty