This can be accomplished in the following way: The input has a voltage follower opamp; it's input is pulled by a relatively large resistor (100k - 1M) to the desired "default" voltage (e.g. 5V). By this, the voltage follower has also this voltage at it's output, when nothing is connected to the input. If a certain CV is connected to the input, this one has a much lower impedance / output resistance, so it overrides the default voltage. Of course this is not 100% perfect, but for usual synth modulation CVs this is good enough (IMO). HTH...!