Telephony: check if the provisioning apn is actually empty before

checking if they are equal. fixes cases where the stubbed apn
(empty) equal to the the empty provisioning apn.

Change-Id: I6ca4fd172e2ecc64ca7a2c3fbca5327be61e7b5c
Merge branch 'cm-10.2' of https://github.com/CyanogenMod/android_frameworks_opt_telephony into jb4.3