fix: advanced marker position can be null

[LeLunZ]

Fixes #902 🦕

Currently there is still a problem with the tests. As google.maps.Markers and google.maps.marker.AdvancedMarkerElement are inconsistent in their way of handling the position. .getPosition on google.maps.Marker returns a LatLng(lat=0, lng=0) even if no position was provided. While google.maps.marker.AdvancedMarkerElement .position really is null.

If a fix like the one provided in the PR would be accepted, the tests needs to be changed because of the inconsistency.