An empty multi-lang `FileBone` has `None` values

```py
 Traceback (most recent call last):
  File "/layers/google.python.pip/pip/lib/python3.11/site-packages/viur/toolkit/importer/importable.py", line 616, in _convert_entry
    assert skel.toDB(update_relations=updateRelations)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/layers/google.python.pip/pip/lib/python3.11/site-packages/viur/core/skeleton.py", line 1193, in toDB
    bone.postSavedHandler(skel, bone_name, key)
  File "/layers/google.python.pip/pip/lib/python3.11/site-packages/viur/core/bones/file.py", line 224, in postSavedHandler
    handleDerives(values[lang])
  File "/layers/google.python.pip/pip/lib/python3.11/site-packages/viur/core/bones/file.py", line 217, in handleDerives
    for val in values:  # Ensure derives getting build for each file referenced in this relation
TypeError: 'NoneType' object is not iterable
```

#1320) (#1351)

Fix for : #1348

---------

Co-authored-by: Jan Max Meyer <jmm@phorward.de>