chore: fix parse typo for network telemetry (#13971)

tailnet/service.go

@@ -261,6 +261,9 @@ func NewNetworkTelemetryBatcher(clk quartz.Clock, frequency time.Duration, maxSi
 		closed:    make(chan struct{}),
 		done:      make(chan struct{}),
 	}
+	if b.batchFn == nil {
+		b.batchFn = func(batch []*proto.TelemetryEvent) {}
+	}
 	b.start()
 	return b
 }