Apply suggestion from @s-heppner

Author: s-heppner

sdk/basyx/aas/adapter/aasx.py

@@ -604,7 +604,8 @@ def _collect_supplementary_file(file_name: str) -> None:
             if isinstance(the_object, model.Submodel):
                 for element in traversal.walk_submodel(the_object):
                     if isinstance(element, model.File):
-                        _collect_supplementary_file(element.value)
+                        if element.value:
+                            _collect_supplementary_file(element.value)
 
         # Add aas-spec relationship
         if not split_part: