[1.7.x] Fixed #23920 -- Fixed MySQL crash when adding blank=True to TextField.

Thanks wkornewald for the report and Markus Holtermann for review.

Backport of 765fa36d57 from master

tests/schema/tests.py

@@ -10,7 +10,7 @@ from django.db.transaction import atomic
 from .models import (Author, AuthorWithDefaultHeight, AuthorWithM2M, Book, BookWithLongName,
     BookWithSlug, BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
     UniqueTest, Thing, TagThrough, BookWithM2MThrough, AuthorTag, AuthorWithM2MThrough,
-    AuthorWithEvenLongerName, BookWeak)
+    AuthorWithEvenLongerName, BookWeak, Note)
 
 
 class SchemaTests(TransactionTestCase):
@@ -423,6 +423,19 @@ class SchemaTests(TransactionTestCase):
         self.assertEqual(columns['name'][0], "TextField")
         self.assertEqual(bool(columns['name'][1][6]), False)
 
+    def test_alter_text_field(self):
+        # Regression for "BLOB/TEXT column 'info' can't have a default value")
+        # on MySQL.
+        new_field = TextField(blank=True)
+        new_field.set_attributes_from_name("info")
+        with connection.schema_editor() as editor:
+            editor.alter_field(
+                Note,
+                Note._meta.get_field_by_name("info")[0],
+                new_field,
+                strict=True,
+            )
+
     def test_alter_null_to_not_null(self):
         """
         #23609 - Tests handling of default values when altering from NULL to NOT NULL.