shift + numkey sets only the modifier

[Avasam]

1. Have numlock on
2. Go set a hotkey
3. "shift + a num key 0-9 or ."

The hotkey is set to only "shift"

This happens because with numlock on, pressing shift will input the numlock off version of the keypad.

We could try to support it and adapt the already very complicated code for numpad keys. But at the very least we should not accept "modifier keys only". And it could be a combo of modifier keys only:

Edit: As part of validating hotkeys, we need to prevent an error from being raised if an empty hotkey is somehow caught:
Avasam#35 (comment)

[Avasam]

Of course this is a bug with keyboard... boppreh/keyboard#564 The numkey down is interpreted as shift up instead. I don't care to even attempt to fix that.

I still added a check for invalid hotkeys (can be expanded if more edge cases come up), which should also catch invalid hotkey loaded from settings file.
Even though LiveSplit supports modifier keys as the last key (as commented in code). Let's not support hotkeys that contains ONLY modifier keys, that's just gonna create problems for the user as those keys are usually long pressed and could be part as many other key combinations.