Issue #2307053 by mondrake: Fixed Rotate effect UI does not allow to set...

Issue #2307053 by mondrake: Fixed Rotate effect UI does not allow to set background color to transparent.

core/modules/image/src/Plugin/ImageEffect/RotateImageEffect.php

@@ -128,7 +128,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
    * {@inheritdoc}
    */
   public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
-    if (!Color::validateHex($form_state->getValue('bgcolor'))) {
+    if (!$form_state->isValueEmpty('bgcolor') && !Color::validateHex($form_state->getValue('bgcolor'))) {
       form_set_error('bgcolor', $form_state, $this->t('Background color must be a hexadecimal color value.'));
     }
   }

core/modules/image/src/Tests/ImageAdminStylesTest.php

@@ -237,6 +237,17 @@ function testStyle() {
         '%style' => $style->label,
       )));
 
+    // Additional test on Rotate effect, for transparent background.
+    $edit = array(
+      'data[degrees]' => 5,
+      'data[random]' => 0,
+      'data[bgcolor]' => '',
+    );
+    $this->drupalPostForm($style_path, array('new' => 'image_rotate'), t('Add'));
+    $this->drupalPostForm(NULL, $edit, t('Add effect'));
+    $style = entity_load_unchanged('image_style', $style_name);
+    $this->assertEqual(count($style->getEffects()), 6, 'Rotate effect with transparent background was added.');
+
     // Style deletion form.
 
     // Delete the style.