Fix broken images for items without ISBNs in search results

Fix broken images for items without ISBNs in search results

The search results code happily sets the src attribute of the cover
image to an empty string (as returned by cleanISBN()) if currentISBN has
not been defined (which is generally the case if an item doesn't have an
ISBN, such as a DVD or a thesis or an old book).

Protect against the empty string and set the src attribute to
'/opac/images/blank.png' to avoid broken image indicators in search
results.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Dan Scott <dan@coffeecode.net>