Fixing bug when xpath is incorrect, it fails for all remaining column…

…s in the group

XmlDocumentConverter/XlsxConverter.cs

@@ -156,7 +156,7 @@ private string GetValue(string xpath, XmlNode parent, XmlNamespaceManager nsMana
                 }
             }
 
-            return null;
+            return string.Empty;
         }
 
         private void ProcessGroup(MappingGroup groupConfig, ExcelFormat excelFormat, XmlNode node, XmlNamespaceManager nsManager)