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.

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,