Can I leave my iPhone 7 charging overnight

No near modern electronics can be over charged, it will stop charging when it reach 100% and stop and then begin again when it fall down under x%

have you read

About the battery and performance of iPhone 11 and later - Apple Support

and maybe 

iPhone Battery and Performance - Apple Support

and Batteries - Maximizing Performance - Apple

It is impossible to overcharge the battery.....because the "charger" is actually in the phone and it will automatically regulate the voltage.