There are instructions in the tracker but I guess maybe some people didn't get it.

Setup

• Create a form with a checkboxes field where one or more of the boxes are checked by default.
• Uncheck all boxes and save the form.
• Open the form again.

Before

After saving with and empty selection, Joomla will reset the field to the default values (in which one or more boxes are checked). So there's no way to select none in this case.

After

You will be able to deselect all.

@test: Works as expected. After applying the patch, multi-checkbox values are handled correctly.

Now, it took me some time to get a good test case, so here is to help out the next guy/girl:

1. Create a field called test in the user table. Replace pvrn4 to your own prefix

   ALTER TABLE `pvrn4_users`
   ADD COLUMN `test` VARCHAR(50) NOT NULL DEFAULT '0' AFTER `requireReset`;

2. Add this code to the file administrator\components\com_users\models\forms\user.xml

   <field name="test"
              type="checkboxes"
              multiple="true"
               default="a,b,c">
           <option value="a">A</option>
           <option value="b">B</option>
           <option value="c">C</option>
           <option value="d">D</option>
           <option value="e">E</option>
           <option value="f">F</option>
       </field>

3. Add this line to the file libraries\joomla\user\user.php after line 709:

       $this->test = json_encode($this->test);

4. Add this line to the file administrator\components\com_users\models\user.php after line 78:

       $result->test = json_decode($result->test);

5. Go to the url: administrator/index.php?option=com_users&view=users
6. Edit an existing user and you will see the checkboxes where a,b,c are pre-checked.
7. Without the patch the value in the test field in the database is a mess
8. After applying the patch, the data in the test field is as expected.

@test Works as expected. Thanks.

Also thanks for the good instructions by @roland-d moving to RTC here