QgsFeatureRequest::NoGeometry is optional for non-geometry-layers

With this patch applied, a default constructed `QgsFeatureRequest` will return all features, regardless if the layer has geometry or not.
Up to now, a feature-iterator acquired with the default request would not return any features. The request had to include the `QgsFeatureRequest::NoGeometry` flag.

The explicit meaning of the `QgsFeatureRequest::NoGeometry` flag is now:
The geometry is not required to satisfy the request. It may still be returned if e.g. required for a filter condition.