correcting a validation issue for 'voyage'

src/server/BreCal/validators/validation_base_utils.py

@@ -1,7 +1,8 @@
+import typing
 from string import ascii_letters, digits
 
 
-def check_if_string_has_special_characters(text:str):
+def check_if_string_has_special_characters(text:typing.Optional[str]):
     """
     check, whether there are any characters within the provided string, which are not found in the ascii letters or digits
         ascii_letters: abcd (...) and ABCD (...)
@@ -11,6 +12,8 @@ def check_if_string_has_special_characters(text:str):
     User: https://stackoverflow.com/users/10035985/andrej-kesely
     returns bool
     """
+    if text is None:
+        return False
     return bool(set(text).difference(ascii_letters + digits + ' '))