RT26522: Don't treat non-available items as a special case

RT26522: Don't treat non-available items as a special case

Previously, when the 'available' element of a Library Availability API
response was undefined, we assumed that the title itself was no longer
part of the library's current Overdrive collection.  Thus, the Place
Hold link was removed and a "No longer available" message was displayed
in the Status column.  In fact, however, it may just be the case that
the item is currently in the collection, but all copies are checked out.
So it's preferable to retain the Place Hold link and show "0 of X
available" in the Status column -- in other words, don't treat undefined
availability as a special case.

Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>