Fixed set_name error when importing OBJ files.

A different method was used to fix the error in
4c69b61, but it had the
unintentional side effect of shifting the names of the meshes over
by one.

Instead, this method simply does not attempt to add a mesh to the
list (upon encountering an "o" command) if a vertex has not been
registered yet. Much nicer :)

editor/import/resource_importer_obj.cpp

@@ -361,7 +361,7 @@ static Error _parse_obj(const String &p_path, List<Ref<Mesh>> &r_meshes, bool p_
  }
 
  if (l.begins_with("o ") || f->eof_reached()) {
- if (!p_single_mesh) {
+ if (!p_single_mesh && vertices.size() > 0) {
  mesh->set_name(name);
  r_meshes.push_back(mesh);
  mesh.instance();