Bug 20625: Handle non existing of active currency

Bug 20625: Handle non existing of active currency

This patch avoid the explosion in the air when creating a new patron
category and no active currency is defined.

Test plan:
Remove the currencies, create a new patron category
Without this patch applied you will get
  Can't call method "p_sep_by_space" on an undefined value

With this patch the form will be displayed correctly and you will be
able to create the patron category

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>