Fixed #31155 -- Fixed a system check for the longest choice when a named group contains only non-string values.

Regression in b6251956b6.

Thanks Murat Guchetl for the report.

django/db/models/fields/__init__.py

@@ -269,10 +269,10 @@ class Field(RegisterLookupMixin):
                 ):
                     break
                 if self.max_length is not None and group_choices:
-                    choice_max_length = max(
+                    choice_max_length = max([
                         choice_max_length,
                         *(len(value) for value, _ in group_choices if isinstance(value, str)),
-                    )
+                    ])
             except (TypeError, ValueError):
                 # No groups, choices in the form [value, display]
                 value, human_name = group_name, group_choices