fix(ods2csv): Get cell's value from 'office:<type>-value' property (refs #4538)

If celldata (extracted from <text:p> nodes) is empty, try to get the
value from the 'office:<type>-value' or 'office:value' properties of the
<table:table-cell> node.
Merge pull request #14 from eguaj/3.2-A4538

fix(ods2csv): Get cell's value from 'office:<type>-value' property (closes #4538)