NPE from bMessage MIME parsing crashes BT

When a Pixel phone receives a MMS that contains only an image and no
text, it will replace the image data with a text string (e.g., filename)
in the bMessage (if the MAP client specified "no attachments" option).
However, the Pixel does not set the charset encoding to UTF-8 in this
case.

When the MAP client receives a bMessage that does not specify a charset
encoding, it will set the message body to NULL, leading to a NPE that
crashes the BT stack when the MAP client tries to parse a NULL string.

This CL introduces fixes at two points, each of which in isolation
prevents the NPE and BT from crashing. First, instead of setting the
message body to NULL when the UTF-8 check fails, it is set to a
non-UTF-8 encoded string, since the previous assumption that MMS will
always be text-only is no longer valid. The end effect from this change
is the user now sees the replacement text string, e.g., filename.

Second, as an additional failsafe against BT crashing, a NULL-check is
added before any string parsing begins. The end effect from this change
is the user will see a message with an empty-string body. Given the
first fix, this second fix should not be triggered.

Tag: #stability

Bug: 144759976
Test: atest BluetoothMapbMessageMimeTest
Test: Send an image-only MMS to the remote device. A notification on the
carkit should appear indicating a message has been received, where the
image has been replaced with a text string.

Change-Id: I9c2907d6fc42594382cc1eebdf0bcd49cdc44cac