Fix error message being printed when importing an OBJ with no surfaces

[Calinou]

An OBJ with no surfaces is valid, and typically happens when you import an OBJ mesh with no associated MTL file.

Previously, this was printed every time you imported such an OBJ:

ERROR: Index p_surface = -1 is out of bounds (surfaces.size() = 0).
   at: set_surface_name (./scene/resources/3d/importer_mesh.cpp:216)

Testing project: test_obj_import_no_surfaces.zip

[akien-mga]

Thanks!