Bug 29803: Prevent local cover image to be hidden if Coce is enabled

Bug 29803: Prevent local cover image to be hidden if Coce is enabled

If Coce is enabled by no image are returned by the service, then no
other image will be displayed (actually un-hidden)

The getURL is returning early if no image is returned by the service and
the "done" flag is never set.

Test plan:
Add a local cover image to a bibliographic record (ktd biblionumber=14
is good)
Set CoceHost = http://coce.bywatersolutions.com
Select all CoceProviders
Enable OpacCoce

=> If no image is returned by Coce then the local image should be
displayed anyway

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>