Does null current means null tension?
The thing to remember with voltage is that it's not an absolute value. It's the difference between potentials at two different points in the circuit - that is why it is also sometimes called potential difference.

Your \$V_S\$ is the the potential difference between that point and ground. Sometimes that will be small, such as the voltage drop across a diode (when an input is LOW). Other times it will be large, governed by the current flow through the load and the pullup resistor.

\$V_S\$ can never be 0V in your example circuit simply because the lowest it can be is the voltage drop across a diode. For a normal silicon diode that is around 0.7V. The only way you could get \$V_S\$ to be 0V would be to connect it directly to ground bypassing the diode. It is then 0V WRT ground because it is itself ground.