Handle null body for MMS in TelephonyBackupAgent

Treat MMS with body set to null as messages with empty body to avoid
crashing with NPE.

Bug: 120261211
Test: 1) atest TelephonyBackupAgentTest
      2) atest SmsBackupRestoreTest
      3) Remove the null check and verify that TelephonyBackupAgentTest
         fails.

Change-Id: Ie575f1b20fd1728ad49fbf133f8f4cc7d1a90796
2 files changed