[1.7.x] Fixed #23761 -- Fixed crash with MySQL validator and db_type is None.

The issue was fixed on master in e9103402c0.

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, [])