[google_maps_flutter_web] Allow Markers with icon:null

[ditman]

Description

The current conversion of our Marker type to gmaps.Marker assumes that marker.icon is never null (internally, it defaults to BitmapDescriptor.defaultMarker), however users can break the default by creating a marker like so:

Marker(markerId: MarkerId('boom'), icon: null);

This PR makes the Marker to gmaps.Marker conversion resilient to the case where the passed icon is null.

Related Issues

• Fixes [google_maps_flutter_web] Custom marker icon throws TypeError flutter#64938

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Breaking Change

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• No, this is not a breaking change.

[cyanglaz]

LGTM % nits.

[ditman]

Addressed PR comments, and rebased branch.