Assign top cell to the formula group if it's still NULL.

This can happen when Excel's original shared formula range is across
multiple columns, in which case we split that into multiple groups (one
per column), and initialize top cell only for the left most column.

This fixes the crasher on the doc from fdo#31296.

Change-Id: I9e4def9836947fc67523f0d99ca981465709b934