Bug 26976: Display 0 if renewalsallowed is not defined

Bug 26976: Display 0 if renewalsallowed is not defined

If the circulation rule "renewalsallowed" is not set (or an empty
string) then the checkout list displays
(0 of  renewals remaining)
It should display "0 of 0"

Test plan:
Set renewalsallowed to an empty string
Check an item out
Notice the change

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>