Bug 35558: Do not retrieve the local image if none exists - OPAC

Bug 35558: Do not retrieve the local image if none exists - OPAC

Test plan:
 1 - Enable system preferences:
     LocalCoverImages
     OPACLocalCoverImages
 2 - open browser tools network page
 3 - Perform an intranet search
 4 - Note a request for each bib like:
    http://localhost:8081/cgi-bin/koha/catalogue/image.pl?thumbnail=1&biblionumber=35
 5 - Repeat on opac, same requests
 6 - Apply patches
 7 - Restart all
 8 - Repeat searches, note no fetches
 9 - Add local cover images to several biblios in the results
10 - Repeat searches
11 - Note the fetches are only for those records with images
12 - Confirm images still load correctly

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>