RTFDocumentImpl::resolveChars: don't send empty text tokens to dmapper

This speeds up importing unicode text a lot. Results of a sample of
chinese text of 5 pages:

- before: 18.351s
- after: 6.692s

writerfilter/source/rtftok/rtfdocumentimpl.cxx

@@ -735,7 +735,7 @@ int RTFDocumentImpl::resolveChars(char ch)
         // set components back to zero
         m_aStates.top().aCurrentColor = RTFColorTableEntry();
     }
-    else
+    else if (!aOUStr.isEmpty())
         text(aOUStr);
 
     return 0;