Fix regression: Treat empty string '' as None data element ID (#312)

fixes:
> ERROR    [kohlrahbi] Could not convert the unfolded AHB to a flat AHB for Prüfidentifikator '15003'
ERROR    [kohlrahbi] Error processing pruefi '15003': The data_element '' does not match re.compile('^\\d{4}$')

introduced in v1.1.0

src/kohlrahbi/unfoldedahb/unfoldedahbtable.py

@@ -46,7 +46,7 @@ def _split_data_element_and_segment_id(value: str | None) -> tuple[str | None, s
  """
  returns the data element id and segment id
  """
- if value is None:
+ if not value: # covers both None and empty string
  return None, None
  datenelement_id: str | None
  segment_id: str | None