_imagemagick_convert_exec shouldn't be called directly

The _imagemagick_convert_exec function shouldn't be called directly. Instead we should call _imagemagick_convert main function.

Why?

Because there can be cases when source doesn't exist and convert process hangs forever. This is already solved(in _imagemagick_convert) with a patch in this imagemagick issue: https://www.drupal.org/project/imagemagick/issues/2404733 →