Blender Git Loki

Fix T68135: Do not return NULL/None when converting an object to a mesh.

Does not make sense in the use-cases of that function, especially since
we don't know whether it is actually due to an error, or some glitch
(like an empty curve).

Think we always want to get a mesh when using either operator
conversion, or the `bpy.data.meshes.new_from_object` function.

Note that an assert was also added to ensure we do try to convert from a
valid 'geometry' object type.