... for Writer frames, which were missing the draw:opacity; turns out
this is because now the the default valued properties are not exported
any more, which causes them to be missing in the property array so the
hard-coded indexes in XMLTextExportPropertySetMapper::handleElementItem()
do not find them.

(regression from c3e49660)

Change-Id: I84b6c09b23a2275161135290f1b0ac9f52546a80