Get debug information for null messages from notifications center

Summary:
Sometimes Wearg writes:
23:10:13 < Wearg> null

This change allows to investigate on what happens.

Ref T1253

Test Plan:
The next null message shouldn't appear on the channel.

Partyline should print the raw notification.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1253

Differential Revision: https://devcentral.nasqueron.org/D1556