Bug 27837: (follow-up) Do not check subfield unless tag defined

Bug 27837: (follow-up) Do not check subfield unless tag defined

This was causing extra warnings when the permanent_location field was
unmapped.

We only need to check if there is a field defined as you can define a mapping
without both a field and subfield, so the existence of one implies the other

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 0e00dc78e4ed01678f037a970487fd871fa3e70f)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>