Replaced deprecated String with OUString in sc/../chgtrack

There only was one use of EMPTY_STRING left in a call to a method that is already converted to OUString

Change-Id: Iaeb273f68d1c76d1d40f903a3f782ba5bc75289d

sc/inc/chgtrack.hxx

@@ -34,7 +34,6 @@
 #include <set>
 #include <stack>
 
-#include <tools/string.hxx>
 #include <tools/datetime.hxx>
 #include <tools/mempool.hxx>
 #include <unotools/options.hxx>

sc/source/core/tool/chgtrack.cxx

@@ -1953,7 +1953,7 @@ void ScChangeActionContent::PutValueToDoc(
                             aDestMark.SetMarkArea( aRange );
                             pDoc->InsertMatrixFormula( aPos.Col(), aPos.Row(),
                                 aRange.aEnd.Col(), aRange.aEnd.Row(),
-                                aDestMark, EMPTY_STRING,
+                                aDestMark, EMPTY_OUSTRING,
                                 ((const ScFormulaCell*)pCell)->GetCode() );
                         }
                         break;