fix(sonarcloud): use None for verification_token_user_full_name w/o user

fix SonarCloud code bug pythonbugs:S2259:
> Fix this attribute access on a value that can be 'None'.
> Attributes should not be accessed on "None" values pythonbugs:S2259

See rule: https://rules.sonarsource.com/python/RSPEC-2259/

Previously obj.user.username was tried to be accessed when obj.user was
None. Now return None when obj.user is None as there's no user, and thus
no username to return either.

refs KK-1416

verification_tokens/admin.py

@@ -4,10 +4,10 @@
 from .models import VerificationToken
 
 
-def verification_token_user_full_name(obj):
+def verification_token_user_full_name(obj: VerificationToken) -> str | None:
     if obj.user is not None:
         return obj.user.guardian.full_name
-    return obj.user.username
+    return None
 
 
 verification_token_user_full_name.short_description = "Name"