fdo#47689: Avoid crashing mdworker: Don't use uninitialized variable

It's the isCustom field that should be checked to see whether the
customAttribute field has been assigned or not.

Change-Id: I5d2af26b675ab0cbc0e1844eb98ebaf5145eb73d
Reviewed-on: https://gerrit.libreoffice.org/7418
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>