Null Check MCC and MNC for CellIdentity Location

In CDMA the MCC and MNC can be null, and the existing
checks for invalid PLMN didn't actually check for
invalid PLMN. Add a simple set of null checks as the
smallest possible change.

Bug: 141416587
Test: manual with CDMA
Change-Id: Ib34fa65fefd7fc17ee77e698b1269f8c40ce5688