document when we are going to create invalid OOXML documents

That makes my life so much easier when I need to debug where we screw
up.

Change-Id: Ia0b54eef47a06c43e3d0ba66b909e398e02a6d0e

sc/source/filter/excel/xelink.cxx

@@ -1390,7 +1390,10 @@ void XclExpXct::SaveXml( XclExpXmlStream& rStrm )
 {
     XclExpCrnList aCrnRecs;
     if (!BuildCrnList( aCrnRecs))
+    {
+        SAL_WARN("sc", "generating invalid OOXML files: sheetDataSet without sheetData child element");
         return;
+    }
 
     sax_fastparser::FSHelperPtr pFS = rStrm.GetCurrentStream();