Simplify XSLs by using new pz:metadata attribute 'empty'.

Follow Index Data’s commit 579b3142b890d13a8524c0fb847081b632fc25ac

Depends on pazpar2 1.6.26+

http://git.indexdata.com/?p=pazpar2.git;a=commit;h=579b3142b890d13a8524c0fb847081b632fc25ac