XLIFF: format empty translation as <target></target>

Change the code to output empty translations as <target></target>
instead of <target/> because the latter is unexpected and possibly
non-conformant.

src/catalog_xliff.cpp

@@ -638,6 +638,9 @@ class XLIFF12CatalogItem : public XLIFFCatalogItem
         else // no translation
         {
             remove_all_children(target);
+            // Ensure the node is shown as <target></target> rather than <target/>.
+            // The spec is unclear in this regard and the former is more expected.
+            target.text() = "";
         }
 
         Impl_UpdateTargetState(target, !trans.empty(), m_isFuzzy);
@@ -821,6 +824,9 @@ class XLIFF2CatalogItem : public XLIFFCatalogItem
             m_node.remove_attribute("state");
             m_node.remove_attribute("subState");
             remove_all_children(target);
+            // Ensure the node is shown as <target></target> rather than <target/>.
+            // The spec is unclear in this regard and the former is more expected.
+            target.text() = "";
         }
     }