Bug 24800: Handle inexistent return date on self checking

Bug 24800: Handle inexistent return date on self checking

See bug 24800 comment 0 for a description of the problem.

We do not want the SIP server to crash if it receives a checkin request
with a return date that is not given.

The option this patch chose is to parse it only if provided.

Signed-off-by: Clemens Elmlinger <clemens.elmlinger@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>