updates tailscale/corp#40994
If a watcher doesn't also set NotifyInitialNetMap, they can end up receiving
deltas temporarily without any base netmap to which to apply them to.
This change ensures that there is no requirement to set both flags. There
is no scenario in which a caller doesn't need the base netmap in the first response.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>