Bug 16240: (follow-up for 16082) Do not display message if no borrowernumber passed

Bug 16240: (follow-up for 16082) Do not display message if no borrowernumber passed

"This patron does not exist" message should not be displayed on the
"Checkouts home page". The message should only be displayed if the
borrowernumber parameter is passed.

Test plan:
Go on circ/circulation.pl
=> No message
Go on circ/circulation.pl?borrowernumber=424242
=> You should see the message

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>