Fix: filter_difficulty now handles when trek has no difficulty value

geotrek/trekking/parsers.py

@@ -667,6 +667,7 @@ def filter_source(self, src, val):
 
     def filter_difficulty(self, src, val):
         types_clientele = val
+        difficulty_level = None
         for tc in types_clientele:
             if tc['id'] in self.types_clientele_ids_as_difficulty_levels:
                 difficulty_level = tc