update-test-path: Fix LD_LIBRARY_PATH setup

This script fails when LD_LIBRARY_PATH is not set before calling it and
calling script has `nounset` option set.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

update-test-path

@@ -12,5 +12,5 @@ aarch64) ARCHDIR=arm64 ;;
 esac
 
 PATH=${TESTPATH}/${ARCHDIR}/bin:$PATH
-export LD_LIBRARY_PATH=${TESTPATH}/${ARCHDIR}/lib:$LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=${TESTPATH}/${ARCHDIR}/lib:${LD_LIBRARY_PATH:-}