Mms: Add null check for mmp

CM devices that set config_mms_content_disposition_support to false create an instance where pdu
headers are ignored.

See:
https://github.com/android/platform_frameworks_base/blob/master/core/res/res/values/config.xml#L804

Change-Id: I33b7c6878468208867a9e96133655cb7109f6415
Check if mmp's encoded subject is null before used

From AOSP:
  https://android-review.googlesource.com/#/c/57709/1

Change-Id: I1de3c83b2b68128f725f8bbaf3d6764b2bdf836b
Signed-off-by: Roger Chen <cxr514033970@gmail.com>