NetworkMap text diffs being empty were currently used to short-circuit
calling magicsock's SetNetworkMap (via Engine.SetNetworkMap), but that
went away in c7582dc2 (0.100.0-230)
Prior to c7582dc2 (notably, in 0.100.0-225 and below, down to
0.100.0), a change in only disco key (as when a node restarts) but
without endpoint changes (as would happen for a client not behind a
NAT with random ports) could result in a "netmap diff: (none)" being
printed, as well as Engine.SetNetworkMap being skipped, leading to
broken discovery endpoints.
c7582dc2 fixed the Engine.SetNetworkMap skippage.
This change fixes the "netmap diff: (none)" print so we'll actually see when a peer
restarts with identical endpoints but a new discovery key.
6298018704