[lib] avoid creating notificationsCreationData on DMOperation received from other peer
Summary: Looks like we're generating notificationsCreationData even when DMOperation was received, later on this is discarded, but we can avoid generating data that is dropped.
Test Plan:
- No data generated when DMOperation is received.
- Notifs works
Reviewers: tomek, marcin
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D13388