If - for some reason - the name in a scene collection is empty (or
the key doesn't even exist), use the filename stem as a fallback name.
This ensures that the scene collection entry in the cache has a valid
non-empty name.
Using an std::string allows the use of its "empty" method to check for
this otherwise invalid state of the collection data.
Empty names for scene collections are not allowed, but this constraint
is only enforced by UI code during text input.
Thus frontend API users are able to create valid scene collections with
empty names which will lead to later possible crashes as the scene
collection is malformed.
As the frontend API uses the related function CreateNewSceneCollection
which handles exceptions by this method, no crashes should occur when
an empty string is provided by an API caller.
