ril: avoid sending null aid string in getIMSI

A change to how getIMSI works in JB causes null aid strings
to be sent.  The result for RILs older than v7 is a corrupt
ril response in SIMRecords::fetchSimRecords.  This corruption
ends up setting mImsi to null (in SIMRecords) and that causes
havoc w/ 3g/4g data connections.

This patch avoids sending null aid strings and has fixed
data connection issues on several Motorola devices.

Change-Id: Ic1fc363e35d79793c48d19080a2e028e428b888f