Drupal 8 migrating image from local folder per XML file
Thanks to quietone, I figured it out. The settings for skipping row didn't work as expected. Instead of repairing, it's much easier for me to modify the XML-file and remove all rows without image node.

So I don't need the skipping plugin anymore and can change the process part to this (and it's working):

process:
  file_source:
  -
    plugin: concat
    delimiter: /
    source:
      - constants/file_source_uri
      - clio_cover
  file_dest:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/file_dest_uri
        - clio_cover
  uid:
    plugin: default_value
    default_value: '1'
  uri:
    plugin: file_copy
    source:
      - '@file_source'
      - '@file_dest'
  filename: clio_cover_name