Sending empty strings instead of NULL for some RIL requests

Sending empty strings instead of NULL for some RIL requests

Some vendor RILs expect empty string instead of NULL pointer
for certain RIL requets. For RIL_REQUEST_ENTER_SIM_PIN,
RIL_REQUEST_ENTER_SIM_PUK, RIL_REQUEST_ENTER_SIM_PIN2,
RIL_REQUEST_ENTER_SIM_PUK2, RIL_REQUEST_CHANGE_SIM_PIN,
RIL_REQUEST_CHANGE_SIM_PIN2,
RIL_REQUEST_ENTER_NETWORK_DEPERSONALIZATION
RIL_REQUEST_SETUP_DATA_CALL,
RIL_REQUEST_SET_INITIAL_ATTACH_APN, RIL_REQUEST_SET_DATA_PROFILE,
RIL_REQUEST_ENTER_NETWORK_DEPERSONALIZATION,
RIL_REQUEST_SET_FACILITY_LOCK,
RIL_REQUEST_CHANGE_BARRING_PASSWORD, sending empty string instead
of NULL.

Test: Manual
Bug: 37782683, 65653485
Change-Id: I6627c8a12e096b9ac8ebe08e734782cfa951c488