Update the success logic to handle additional cases.

When an image is edited using a target other than the
all and the thumbnail make sure the `success` method
returns the correct value.

tests/testdata/modules/images/webp-uploads/class-wp-image-edit.php

@@ -162,6 +162,16 @@ public function save() {
 * @return bool whether the operation to save the image was succesfully or not.
 */
 public function success() {
-return is_object( $this->result ) && property_exists( $this->result, 'msg' ) && property_exists( $this->result, 'thumbnail' ) && 'Image saved' === $this->result->msg;
+if ( ! is_object( $this->result ) ) {
+return false;
+}
+
+$valid_target = true;
+// The thumbnail property is only set in `all` and `thumbnail` target.
+if ( 'all' === $this->target || 'thumbnail' === $this->target ) {
+$valid_target = property_exists( $this->result, 'thumbnail' );
+}
+
+return property_exists( $this->result, 'msg' ) && $valid_target && 'Image saved' === $this->result->msg;
 }
 }