[google_maps_flutter] TileOverlay fix - null tileProvider caused TileOverlay to not be displayed on the map

[liorboyango]

Added tileProvider to TileOverlay#copyWith - this fixes tileProvider not being cloned inside the TileOverlay object, which broke the TileOverlay;

Fixed a runtime cast exception in the example tile_overlay.dart;

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[liorboyango]

@googlebot I signed it!

[stuartmorgan-g]

Thanks for the contribution! Please work through the checklist on the PR template, as this change is missing a number of the things laid out there (tests, issue link, CHANGELOG and version update). Once you've done that this can be reviewed.

[stuartmorgan-g]

Since there's been no activity on addressing the comment above, I've fixed this in #3771. Closing in favor of that. Thanks again for the clear issue report and root-causing!