Baking completes seemingly without errors even when there is no light or no model with enabled BakedLightmap instance

If there are no lights set up for baking, the baking should be canceled and reason reported in callback. The number of lights is reported to be zero in a progress report, but the baking continues anyway.

Similarly, if no models have enabled BakedLightmap instances attached to them, baking should be canceled and reason reported in callback. This problem is very hard to notice from the callback log, especially for new users of baking.

By canceling the baking with a clear reason in these two cases allows user to better troubleshoot the issue.