[DO NOT MERGE] Check CellInfoCallback Detail for Null

In cases where the source of the CellInfo fails
but does not generate a Throwable, the detail info
will not be provided and should be null. Inside the
user process, we need to check for null before
dereferencing the detail object. Oops. :-(

Bug: 142706765
Bug: 142421696
Test: manual
Change-Id: I1307d1d7aacef0bbd33bfc73700ee774e12e64c1