Bug 18540: (QA follow-up) Only mention index_sort_title if defined

Bug 18540: (QA follow-up) Only mention index_sort_title if defined

This patch makes the generated xslt not mention index_sort_title unless
the entry is defined on the xml file. Otherwise there's a call to

<xslo:apply-templates mode="index_sort_title"/>

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit af9081f9e12ac1a3165e425a7991604a39b917fb)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>