evdev: improve capslock detection for no led device

[yuxqiu]

The current Caps Lock detection does not work correctly on my Sofle keyboard: the state instantly flips from true back to false.

After checking the logs, the cause is clear:

if event.Type == evKeyType && event.Code == keyCapslockKey && event.Value == keyStateOn {
    time.Sleep(50 * time.Millisecond)
    m.readAndUpdateCapsLockState(deviceIndex) // triggered here
} else if event.Type == evLedType && event.Code == ledCapslockKey {
    capsLockState := event.Value == keyStateOn
    m.updateCapsLockStateDirect(capsLockState)
}

and inside the called function:

ledStates, err := device.State(evLedType)
...
capsLockState := ledStates[ledCapslockKey] // root cause

My keyboard has no physical Caps Lock LED, so device.State(EV_LED) returns an empty map.
Accessing it yields false, which forces the state back to off every time.

This PR adds a simple check: if ledStates is completely empty, we skip the update entirely instead of treating Caps Lock as off.

The change has minimal impact on other devices:

• When ledStates is non-empty (normal case), behaviour is unchanged.
• When it is empty, we now avoid the incorrect forced-off state that some keyboards currently suffer from.

Actually, I wonder if it is even necessary to have the branch below since it seems EV_LED event will always be issued by the kernel?

if event.Type == evKeyType && event.Code == keyCapslockKey && event.Value == keyStateOn {
    time.Sleep(50 * time.Millisecond)
    m.readAndUpdateCapsLockState(deviceIndex) // triggered here
}