mapper: fix empty Peers list not clearing client peer state

When a FullUpdate produces zero visible peers (e.g., a restrictive
policy isolates a node), the MapResponse has Peers: [] (empty
non-nil). The Tailscale client only processes Peers as a full
replacement when len(Peers) > 0 (controlclient/map.go:462), so an
empty list is silently ignored and stale peers persist.

This triggers when a FullUpdate() replaces a pending PolicyChange()
in the batcher. The PolicyChange would have used computePeerDiff to
send explicit PeersRemoved, but the FullUpdate goes through
buildFromChange which sets Peers: [] that the client ignores.

When a full update produces zero peers, compute the peer diff
against lastSentPeers and add explicit PeersRemoved entries so the
client correctly clears its stale peer state.