Previously there was no check to make sure that an issn number

Previously there was no check to make sure that an issn number
actually existed; so if there was no isbn, and the issn was blank,
the item would be assigned a random biblionumber and the breeding farm
would report that the item already exists in the catalog (even though
it didn't). This fix adds a check to determine whether the imported
record has an issn before assigning a matching biblionumber.