OpenXML: text runs containing only <w:br/> lost during merge

Testcase attached. This file contains several lines with forced line breaks (<w:br/>) used as spacing. These line breaks are stored in their own dedicated text runs.

To reproduce, roundtrip it through OpenXML filter, eg:

#!bash
tikal.sh -fc okf_openxml -x br.docx
tikal.sh -fc okf_openxml -m br.docx.xlf

In the roundtripped file, these line break runs have been stripped, and the spacing of the document is messed up.