Web UI button fix

[pkendall64]

Checks if the button entry is undefined before trying to store data in it.
Also fixes the feedback dialog when saving the button actions.

Fixes #2162

Testing

1. Use a TTGO (or other TX module with a single button or comment out the second button definition in the Radiomaster Ranger JSON file)
2. Boot into wifi mode, go to the button tab and press the save button. There should not an error in the web-console and you should see the success dialog.

[JyeSmith]

I dont currently have my Ranger to test 😞

[schugabe]

With the missing } it fixes the issue

[CapnBry]

master

• Flashed master to Ranger, verified both buttons / colors working, able to save values and persist across reboots. Do not get any confirmation popup when saving values. :( ❌
• Removed "Button 2 pin" and "Button 2 RGB Index" from hardware.html
• Button 2 now missing from webui, but can't save Button 1's stuff due to "Uncaught type error" in javscript ❌

PR

• Flashed PR code, can save single button and single LED settings. Do get confirmation popup. ✔️
• Reset hardware config, can save 2 button and LED settings. Do get confirmation popup ✔️
• Also tried to trick it by removing "Button 2 pin" but leaving "Button 2 RGB Index". Does not throw any errors saving. The RGB LED still works, but obviously no way to change the value. Get another button, fool! ✔️