Which can happen if an (anonymous) database range as filter range was selected
to span an entire column of which most (the tail) is empty cells. The entries
are processed to be unique anyway. This significantly shortens the time to
display the filter dialog.

Happened with attachment
http://bugs.documentfoundation.org/attachment.cgi?id=136074 of bug
https://bugs.documentfoundation.org/show_bug.cgi?id=112258

Change-Id: Ia169911d6f23611a3b3ecbf291757465f1998210
Reviewed-on: https://gerrit.libreoffice.org/42118Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>