Bug 26797: Escape firstname to prevent relative's checkouts to crash

Bug 26797: Escape firstname to prevent relative's checkouts to crash

If no firstname is defined for a relative, the "Relative's checkouts"
tab will be "processing" forever.
JS error:
  TypeError: oObj.borrower.firstname is null

Test plan:
1 - Create a patron (PatronA) in a category with type 'Org'
    'School' in sample data works
2 - Add a guarantee (Patron B)
3 - Check out an item to PatronA
4 - Go to PatronB's record and click relativwe's checkouts
5 - There is an error and the table doe snot load
6 - Apply patch
7 - Reload
8 - Table loads correctly

Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 13fb0906be41e6f2f5effb89321bcac8371c8b45)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>