Bug 25747: Don't display a comma when patron has no firstname

Bug 25747: Don't display a comma when patron has no firstname

This patch adds a check for the existence of "firstname" to
patron-title.inc so that if the patron has no firstname the "invert
name" setting doesn't cause an unnecessary comma to appear.

To test, apply the patch and locate or create a patron with no first
name.

- When checking out to the patron, examine the page title. It should say
  "Checking out to <lastname> (<cardnumber>).
- When checking out to a patron with first and last name it should show
  as before, "Checking out to <lastname>, <firstname> (<cardnumber)

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>