bt xml: empty strings are now exported as <string></string> no longer <string/>

bt5/erp5_km_ui_test_data/PathTemplateItem/knowledge_pad_module/erp5_km_public_front_pad/6.xml

@@ -131,7 +131,7 @@
         </item>
         <item>
             <key> <string>preferred_password</string> </key>
-            <value> <string/> </value>
+            <value> <string></string> </value>
         </item>
         <item>
             <key> <string>preferred_rss_feed</string> </key>
@@ -139,7 +139,7 @@
         </item>
         <item>
             <key> <string>preferred_username</string> </key>
-            <value> <string/> </value>
+            <value> <string></string> </value>
         </item>
         <item>
             <key> <string>title</string> </key>

bt5/erp5_km_ui_test_data/PathTemplateItem/web_site_module/predecessor_test_web_site/discussions.xml

@@ -164,7 +164,7 @@
             <key> <string>membership_criterion_category</string> </key>
             <value>
               <tuple>
-                <string/>
+                <string></string>
               </tuple>
             </value>
         </item>