B1G Harvest: image_map_layer white images

[ewlarson]

Some image_map_layer results are just "white" boxes:

[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [remote_content_type] image/png
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [viewer_protocol] image_map_layer
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [image_url] http://gis.hennepin.us/arcgis/rest/services/Imagery/UTM_Aerial_2006/MapServer/info/thumbnail/thumbnail.png
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [gblsi_thumbnail_uri] 
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [service_url] http://gis.hennepin.us/arcgis/rest/services/Imagery/UTM_Aerial_2006/MapServer/info/thumbnail/thumbnail.png
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [image_extension] .png
[ab12ccd1-9ce1-4274-baf6-739276a3aa8b] [sidecar_image_url] /uploads/solr_document_sidecar/image/919/ab12ccd1-9ce1-4274-baf6-739276a3aa8b20180321-57032-yootz6.png

See if we can detect that? Or add a way to whitelist/blacklight collections from harvest?