Fixed #23761 -- Added test for MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0fa873aea58a6a11dba510cd308cb84
so this just forwardports the test and release notes from stable/1.7.x.

Forwardport of 2d12a59938 from stable/1.7.x

docs/releases/1.7.2.txt

@@ -51,8 +51,11 @@ Bugfixes
   (:ticket:`23420`).
 
 * Fixed a migration serializing bug involving ``float("nan")`` and
-  ``float("inf")`` (:ticket:23770:).
+  ``float("inf")`` (:ticket:`23770`).
 
 * Fixed a regression where custom form fields having a ``queryset`` attribute
   but no ``limit_choices_to`` could not be used in a
   :class:`~django.forms.ModelForm` (:ticket:`23795`).
+
+* Fixed a custom field type validation error with MySQL backend when
+  ``db_type`` returned ``None`` (:ticket:`23761`).

tests/invalid_models_tests/test_custom_fields.py

@@ -0,0 +1,20 @@
+from django.db import models
+
+from .base import IsolatedModelsTestCase
+
+
+class CustomFieldTest(IsolatedModelsTestCase):
+
+    def test_none_column(self):
+        class NoColumnField(models.AutoField):
+            def db_type(self, connection):
+                # None indicates not to create a column in the database.
+                return None
+
+        class Model(models.Model):
+            field = NoColumnField(primary_key=True, db_column="other_field")
+            other_field = models.IntegerField()
+
+        field = Model._meta.get_field('field')
+        errors = field.check()
+        self.assertEqual(errors, [])