Don't write empty value when exporting grab-bagged w:docPartUnique

See http://dev-builds.libreoffice.org/crashtest/e658cb4d5ce49d3a3c6acc63155974b5ff8490c7/validation/docx/fdo33590-2.docx.log

(footer1.xml:2 col:890) cvc-datatype-valid.1.2.1: '' is not a valid value for 'boolean'.
(footer1.xml:2 col:890) cvc-attribute.3: The value '' of attribute 'w:val' on
element 'w:docPartUnique' is not valid with respect to its type, 'ST_OnOff'.

Original document:
      <w:docPartObj>
        <w:docPartGallery w:val="Page Numbers (Bottom of Page)"/>
        <w:docPartUnique/>
      </w:docPartObj>

RT document:
      <w:docPartObj>
        <w:docPartGallery w:val="Page Numbers (Bottom of Page)"/>
        <w:docPartUnique w:val=""/>
      </w:docPartObj>

Change-Id: If41af8f4bca99fba4d3ad0c966c2345f87ea422a
solution: if value is empty, export it as true (default ST-OnOff value)
Reviewed-on: https://gerrit.libreoffice.org/15686Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/filter/ww8/docxattributeoutput.cxx

@@ -8004,7 +8004,12 @@ void DocxAttributeOutput::ParaGrabBag(const SfxGrabBagItem& rItem)
                                            FSNS( XML_w, XML_docPartCategory ),
                                            OUStringToOString( sValue, RTL_TEXTENCODING_UTF8 ).getStr() );
                         else if (aGrabBag[j].Name == "ooxml:CT_SdtDocPart_docPartUnique")
-                            AddToAttrList( m_pParagraphSdtPrTokenChildren, FSNS( XML_w, XML_docPartUnique ), "" );
+                        {
+                            if (sValue.isEmpty())
+                                sValue = "true";
+                            AddToAttrList( m_pParagraphSdtPrTokenChildren, FSNS( XML_w, XML_docPartUnique ),
+                            OUStringToOString( sValue, RTL_TEXTENCODING_UTF8 ).getStr() );
+                        }
                     }
                 }
                 else if (aPropertyValue.Name == "ooxml:CT_SdtPr_equation")