Fix one NPE in Mms app

For tablet we usually declare not support mobile_* networkAttributes but ethernet,
wifi,bt in config.xml. this will trigger this NPE.

Change-Id: I761c8514f5569bef1242f9fddc321c0aed564d9e
Signed-off-by: Jianzheng Zhou <jianzheng.zhou@freescale.com>