dbmgr: consistently initialize colum type to DataType::SQLNULL

Fixes a warning about use uninitialized in
SwDBManager::FillCalcWithMergeData().

Change-Id: I03954f6d47a699e3a694269757ad9613b92c5695
Reviewed-on: https://gerrit.libreoffice.org/11740
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
Tested-by: Jan-Marek Glogowski <glogow@fbihome.de>