How to make a command block output a negative signal
Solution 1:
One way to do this is to have a second command block testing if the previous one failed:
The impulse command block would have your current command (/testfor @e[type=Zombie]).
The chain command block, which is the one you should run comparators/conditionals off of, would then have:
/testforblock X Y Z command_block -1 {SuccessCount:0}
X Y Z should be the coordinates of the impulse command block.
This makes the chain block succeed when the impulse block fails.