JSON-LD thumbnail issue

[coret]

When I look at the JSON-LD from an Item (like https://www.goudatijdmachine.nl/omeka/api/items/92296) I notice that the o:thumbnail property has a null value, and the thumbnail_display_urls property (which is missing a context, like o: !!) does contain URL's of thumbnails:

"o:thumbnail": null,
"thumbnail_display_urls": {
  "large": "https://www.goudatijdmachine.nl/omeka/files/large/af173459f99f48fc069fd9d55ad039f400d42b35.jpg",
  "medium": "https://www.goudatijdmachine.nl/omeka/files/medium/af173459f99f48fc069fd9d55ad039f400d42b35.jpg",  
  "square": "https://www.goudatijdmachine.nl/omeka/files/square/af173459f99f48fc069fd9d55ad039f400d42b35.jpg"
},

Is this an incorrect configuration of my Omeka S instance or is there an error in Omeka S code in

omeka-s/application/src/Api/Representation/AbstractResourceEntityRepresentation.php

Lines 126 to 128 in 183904e

	'o:thumbnail' => $thumbnail,
	'o:title' => $this->title(),
	'thumbnail_display_urls' => $this->thumbnailDisplayUrls(),

[zerocrates]

o:thumbnail is specifically for a custom thumbnail set via an asset, not the generated thumbnails, so it's expected for it to be null unless you've overridden the thumbnail.

My recollection on thumbnail_display_urls is that it omits the context purposely to allow that array format, but that could be changed (or duplicated) if necessary.