fdo#76586 Table column separators from individual cell widths

In the test file Table Grid had only one value for entire table width
while there are two cells in a table row. So the table was not
imported with the correct Column Separators.

Added fix to calculate column seperators from cell widths if
unable to calculate from Table Grid, and all the cells have individual cell widths and
there are no after and before cells.

Change-Id: I466737437a18d3373e75fea5ad56c9e31eb149ec
Reviewed-on: https://gerrit.libreoffice.org/8767
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>