Fixed special_attributes_str loop only it items are available

At issue #323 the ad_2827968602 had NO special_attributes. 
This broke down the for loop.

Fixed by adding a check for tiems.

src/kleinanzeigen_bot/extract.py

@@ -290,7 +290,7 @@ async def _extract_special_attributes_from_ad_page(self) -> dict[str, Any]:
  """
  belen_conf = await self.web_execute("window.BelenConf")
  special_attributes_str = belen_conf["universalAnalyticsOpts"]["dimensions"]["dimension108"]
- special_attributes = dict(item.split(":") for item in special_attributes_str.split("|"))
+ special_attributes = dict(item.split(":") for item in special_attributes_str.split("|") if ":" in item)
  special_attributes = {k: v for k, v in special_attributes.items() if not k.endswith('.versand_s') and k != "versand_s"}
  return special_attributes