OpenXML Filter: PPTX: merged document corrupted over the appearance of an incomplete initial run before a line break

The issue is reproducible under bPreferenceAddLineSeparatorAsCharacter condition set to true.

When a translation is leaded by a \n character (or any other specified by the sPreferenceLineSeparatorReplacement parameter), the following structure shows up:

         <a:p>
            <a:r>
            </a:r>
            <a:br/>

This is not allowed, according to the specification and has to be improved to contain an empty text:

         <a:p>
           <a:r>
              <a:t></a:t>
            </a:r>
            <a:br/>

A sample document can be found attached.