[1.8.x] Fixed #26557 -- Converted empty strings to None when saving GenericIPAddressField.

Backport of 4681d65048 from master

django/db/backends/base/operations.py

@@ -481,7 +481,7 @@ class BaseDatabaseOperations(object):
         Transforms a string representation of an IP address into the expected
         type for the backend driver.
         """
-        return value
+        return value or None
 
     def year_lookup_bounds_for_date_field(self, value):
         """

docs/releases/1.8.13.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed ``TimeField`` microseconds round-tripping on MySQL and SQLite
   (:ticket:`26498`).
+
+* Restored conversion of an empty string to null when saving values of
+  ``GenericIPAddressField`` on SQLite and MySQL (:ticket:`26557`).

tests/model_fields/tests.py

@@ -722,6 +722,14 @@ class GenericIPAddressFieldTests(test.TestCase):
         o = GenericIPAddress.objects.get()
         self.assertIsNone(o.ip)
 
+    def test_blank_string_saved_as_null(self):
+        o = GenericIPAddress.objects.create(ip='')
+        o.refresh_from_db()
+        self.assertIsNone(o.ip)
+        GenericIPAddress.objects.update(ip='')
+        o.refresh_from_db()
+        self.assertIsNone(o.ip)
+
     def test_save_load(self):
         instance = GenericIPAddress.objects.create(ip='::1')
         loaded = GenericIPAddress.objects.get()