Why isn't \$I_s\$ (resultant current phasor) zero in FOC of PMSM?
The sum of electrical current flowing through three wires is \$I_a+I_b+I_c = 0 \$, so still zero. I can't be otherwise, since as much current goes in one direction it has to return back in opposite direction, this is basic Kirchhoff law.

The \$I_s\$, stator current is the vector sum of all three currents, which are phase shifted by 120 degrees and also geometrically shifted by 120 degrees. What you get is a rotating stator current, search google about rotating magnetic field in the AC motor.