param: fix potential nullptr dereferencing on param import
param: fix potential nullptr dereferencing on param import

NuttX 7.28 seemed to handle this gracefully, but officially passing NULL
results in undefined behavior, and with 7.29 leads to a hardfault.

This happens on configs with flash-based params, on the first unsuccessful
import attempt.

krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware