[rilmodem] numeric value can be null in response to RIL_REQUEST_OPERATOR

According to ril.h numeric value can be null in response to
RIL_REQUEST_OPERATOR. That means phone is unregistered and
current operator should return error

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #114 from jkangas/master

Request for pulling operator name numeric value check to main branch