Specific_impulse returns 0 when throttle is 0, but Vacuum and sea_level impulses return non-zero values.
How to reproduce:

1. Turn engines on to make them active
2. Put throttle to 0

import krpc

conn = krpc.connect(name='Test1')

vessel = conn.space_center.active_vessel
print(vessel.specific_impulse, vessel.vacuum_specific_impulse, vessel.kerbin_sea_level_specific_impulse)

0.0 311.8999938964844 208.50001525878906

Process finished with exit code 0