Allow getRating to return null

[csrui]

The getRating() return signature is not according to the documentation.
This change would fix this.

[jbelien]

Hello @csrui !
Thank you for your contribution 👍

This is a read-only repository.
Could you submit your PR to our main repository: https://github.com/geocoder-php/Geocoder/tree/master/src/Provider/GoogleMapsPlaces ?
Thanks.

[csrui]

I see you already have geocoder-php/Geocoder#1073 that would fix this.