Skip writing message field in an atom if it's empty

Test: unit test added
Bug: 120635548
Change-Id: I825b1ce526944a20fe65705508ad180ece37492c
Merged-In: I825b1ce526944a20fe65705508ad180ece37492c
(cherry picked from commit 8e6f9983009eadf32d05ad65e38906d3985bc9ba)
4 files changed