[5.0.x] Refs #30446, Refs #34944 -- Fixed crash when adding GeneratedField with string Value().

This should allow smarter output_field inferring in functions dealing
with text expressions.

Regression in f333e3513e.

Backport of 8b1acc0440 from main

tests/expressions/tests.py

@@ -2227,6 +2227,7 @@ class ValueTests(TestCase):
             with self.subTest(type=type(value)):
                 expr = Value(value)
                 self.assertIsInstance(expr.output_field, output_field_type)
+        self.assertEqual(Value("foo").output_field.max_length, 3)
 
     def test_resolve_output_field_failure(self):
         msg = "Cannot resolve expression type, unknown output_field"