Map file might not get generated at editor launch if the project contains no scene assets

[starikcetin]

Which will in turn lead to null ref exceptions while trying to access the map.

We can employ two solutions (ideally both):

1. Generate an empty map in those cases.
2. Never return null from map providers, instead, return an empty map.

[starikcetin]

Fixed in #91.
Included in 3.2.1.