Bug 28764: (QA follow-up) Add comment, remove two defined's

Bug 28764: (QA follow-up) Add comment, remove two defined's

If we no longer offer the sortfield with :asc actively, we should
probably write it somewhere too.
Replaced a few defined tests by empty test. No need to split empty
string or zero etc.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit eed750ba95de3aa7dffce0a73073099d40c40a2e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>