\Drupal\imagick\Plugin\ImageToolkit\ImagickToolkit::isValid() should be more restrictive

Problem/Motivation

\Drupal\imagick\Plugin\ImageToolkit\ImagickToolkit::isValid() currently returns TRUE as soon as there is a valid path to the file. The \Drupal\Core\ImageToolkit\ImageToolkitInterface::isValid() interface says that the returned value should be "TRUE if the image toolkit is currently handling a valid image, FALSE otherwise".

This condition might be used by Core or contrib to test if an image is valid or not (see \Drupal\Core\Image\Image::isValid()) so we have to be more restrictive on our implementation.

Proposed resolution

Follow Core and use $this->resource like it's done in \Drupal\system\Plugin\ImageToolkit\GDToolkit::isValid()