Bug 27753: (QA follow-up) Check that item exists before using it

Bug 27753: (QA follow-up) Check that item exists before using it

We're actioning the change of claim status outside of any block that
checks the $item exists. As such we'll want to add that check in here.

I did consider that this should live inside AddIssue, but on reflection
the librarian may want to not proceed with the issue given other return
values from the CanBookBeIssued call, but you still want the
AutoClaimReturn to fire regardless as you've now found the item.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>