Bluetooth: Map: Validate Mime part element while encoding email.

A case where getMessage is performed from remote, NULL pointer
exception is coming if we try to encode email when mime part
element is null. Add change to validate mime part while encoding
email data to fix this issue.

CRs-fixed: 613755
Change-Id: I1cca1875db06ab0b3e2a5c49d8613ec2d18f26a5