Do not add empty semantics to the Geometry when there are no semantics at all

[balazsdukai]

When creating a Geometry instance through the API with geom = Geometry(lod=1), and empty semantics is automatically added to the geometry.
Eg. "semantics": {"surfaces": [], "values": [[null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null]]}

This is unnecessary in this case and increases the size and complexity of the file. Do not do it.