Align empty label implementation with checkbox

[web-padawan]

The following fix for checkbox should be also applied to radio-button: vaadin/vaadin-checkbox@ac40325

Note that the fix from vaadin/vaadin-checkbox#129 should be taken into account and applied too.

I'm not sure how much "critical" this is. Original ticket for checkbox was a BFP but it was related to Grid selection checkbox. Radio-button is less likely to be used without the label.

However, that is still possible (user might want to provide a custom label and handle clicks manually).
So, for consistent DX and to fix not currently working styles, this should be also backported to V10