7453 Followup for borrower without expiry date

7453 Followup for borrower without expiry date

While it should be impossible now to have a patron without expiry date, I still had one. It does not hurt to add a simple check on expiry date defined before calling delta days on null values and breaking the opac login.
Trivial change.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Tested by deleting the expirydate from my own patron record.
Missing expiry date now no longer breaks the summary page
in the patron account on OPAC.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>