Mac menu bar: better distinguish when there are no active devices in Network Devices

### What is the issue?

I expected that hovering over `Network Devices` would show a flyout menu with a list of my devices.
https://user-images.githubusercontent.com/123998549/236003688-0a5c7c17-ce5c-4b92-a892-578ee288bd16.mp4

After some investigating, I realized that I was connected to my personal tailnet where all the other devices were offline.

In this case, it might be nice to still have the flyout menu and add some messaging like `No devices connected` to help users understand what's happening. We do something similar for exit nodes:

### Steps to reproduce

1. Log in to Tailscale
2. Have tailnet where all the other devices are offline
3. Click on Tailscale from the menu bar
4. Hover over `Network Devices`

### Are there any recent changes that introduced the issue?

_No response_

### OS

macOS

### OS version

macOS 13.3.1 (22E261

### Tailscale version

1.38.4 (App Store)

### Other software

_No response_

### Bug report

BUG-10e4fd3baba72e8fa0b8da5cee9940e9c5167f28eee134b9f995d6d13d61ecf2-20230503174706Z-0b4fd01bdf2294ec