We confirm It is normal for the charging pad and the phone to get warm during use; even if there is no device on the pad.
 
Wireless charging uses magnetic induction to charge and the "air" between the devices doesn’t conduct electricity anywhere near as well as a wire does. So, there is going to be some heat in the process.

 
We have run some tests on the iPhone 11 charging, the temperature is about 86°F.