OpenXML Filter: minimise PPTX style definitions as much as possible

It has been found out that some styles can be gradually minified causing the integration tests to fail. For instance, the initial markup can be the following:

<a:defPPr>
    <a:defRPr lang="en-US"/>
</a:defPPr>

So, the first round-trip produces:

<a:defPPr>
    <a:defRPr/>
</a:defPPr>

and the second:

<a:defPPr/>

which leads to a comparison failure.

It would be better to clean up all markup as much as possible at once and get even an empty string in the aforementioned case.