Do not fail to clean when help selection is None

Closes #13862
(grafted from 7459598460593b98dd6a04ecf24551266290e2eb)

--HG--
branch : 6.0

trytond/trytond/ir/translation.py

@@ -1333,7 +1333,11 @@ class TranslationClean(Wizard):
         if not field:
             return True
         help_ = field.help
-        help_selection = list(getattr(field, 'help_selection', {}).values())
+        help_selection = getattr(field, 'help_selection', {})
+        if help_selection:
+            help_selection = list(help_selection.values())
+        else:
+            help_selection = []
         if not help_ and not help_selection:
             return True
         if translation.src not in list(help_) + help_selection: