We appear to be getting one null or empty value in the args.issns array,
which was causing spurious lookups of null ISSNs, so skip the entry if
it is an empty string.
Also, switch from the product-specific "sfx" variable name to the
product-neutral "openurls" as we have CUFTS in the mix these days.
Signed-off-by: Dan Scott <dscott@laurentian.ca> Signed-off-by: Art <artrhyno@uwindsor.ca>