Settings:Fix mImsi null cause ApnSettings crash

mImsi may be "null" in some case,if we use "" instead may
 cause crash in ApnSettings&com.android.phone. so fix it
to use old way to set Apn in that "null"  case.

Ticket: CYNGNOS-3291

Change-Id: I0119e9a3f58c688e0e271ae62f50484ad8a29da9
(cherry picked from commit b4bac984)