simplify and ensure valid miAutoPosColumn in all cases, fdo#78838 related

Unconditionally set miAutoPosColumn at the end of all operations so it
is also valid in case of pColumnData->clear() and no entries inserted
after pDoc->GetDataEntries().

Change-Id: I689632f696091fd9ce8e93c06d7918e5eaf05ab4
(cherry picked from commit 23e9f36a21f7daa6175f53fd244677b9c2e2a660)
Reviewed-on: https://gerrit.libreoffice.org/9412
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>