minor #9804 [Form] Update UPGRADE-2.3.md to account for #9388 (ureimers)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9804).

Discussion
----------

[Form] Update UPGRADE-2.3.md to account for #9388

Added documentation for how to correctly pre-fill a form using the form's `data` option.
The "old" and also wrong way of doing it broke with Symfony 2.3.7 (with #9388 to be precise) and this short documentation should help others to fix the problem and do it right.

Commits
-------

5e06535 [Form] Update UPGRADE-2.3.md to account for #9388

UPGRADE-2.3.md

@@ -170,6 +170,32 @@ Form
    $form = $builder->getForm();
    ```
 
+ * Previously, when the "data" option of a field was set to `null` and the
+   containing form was mapped to an object, the field would receive the data
+   from the object as default value. This functionality was unintended and fixed
+   to use `null` as default value in Symfony 2.3.
+
+   In cases where you made use of the previous behavior, you should now remove
+   the "data" option altogether.
+
+   Before:
+
+   ```
+   $builder->add('field', 'text', array(
+      'data' => $defaultData ?: null,
+   ));
+   ```
+
+   After:
+
+   ```
+   $options = array();
+   if ($defaultData) {
+       $options['data'] = $defaultData;
+   }
+   $builder->add('field', 'text', $options);
+   ```
+
 PropertyAccess
 --------------