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