Ensure graceful behaviour if field_image becomes unavailable

I had a site where field_image ended up being unintentionally disabled due to an issue with the config dependencies when I changed image styles, despite the comment pl9settings@2ebeaa0b

The stack trace has no direct mention of our code but is clearly going through core's render process, hitting e.g. twig_render_template() with the final fatal error being

Error: Call to a member function getCacheTags() on null in Drupal\image\Plugin\Field\FieldFormatter\ImageFormatter->viewElements() (line 241 of $DRUPAL_ROOT/web/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php).