appc: don't schedule advertisement of 0 routes

When the store-appc-routes flag is on for a tailnet we are writing the
routes more often than seems necessary. Investigation reveals that we
are doing so ~every time we observe a dns response, even if this causes
us not to advertise any new routes. So when we have no new routes,
instead do not advertise routes.

Fixes #12593

Signed-off-by: Fran Bull <fran@tailscale.com>

appc/appconnector.go

@@ -442,9 +442,11 @@ func (e *AppConnector) ObserveDNSResponse(res []byte) {
 			}
 		}
 
+		if len(toAdvertise) > 0 {
 			e.logf("[v2] observed new routes for %s: %s", domain, toAdvertise)
 			e.scheduleAdvertisement(domain, toAdvertise...)
 		}
+	}
 }
 
 // starting from the given domain that resolved to an address, find it, or any