Fixed #33406 -- Avoided creation of MaxLengthValidator(None) when resolving Value.output_field for strings.

This brings the behaviour in line with Field subclasses which append to
the validators within __init__(), like BinaryField, and prevents the
creation of a validator which incorrectly throws a TypeError, if it
were used.

django/db/models/fields/__init__.py

@@ -1010,7 +1010,8 @@ class CharField(Field):
     def __init__(self, *args, db_collation=None, **kwargs):
         super().__init__(*args, **kwargs)
         self.db_collation = db_collation
-        self.validators.append(validators.MaxLengthValidator(self.max_length))
+        if self.max_length is not None:
+            self.validators.append(validators.MaxLengthValidator(self.max_length))
 
     def check(self, **kwargs):
         databases = kwargs.get('databases') or []

tests/expressions/tests.py

@@ -1859,6 +1859,7 @@ class ValueTests(TestCase):
         and this demonstrates that they don't throw an exception.
         """
         value_types = [
+            'str',
             True,
             42,
             3.14,