I have added a value(a*1*c*3) to password and then replaced the value with null. You can add a value or set of characters which don't occur regularly to avoid any data issues
select
REPLACE((XMLROOT(
XMLELEMENT("prod",
XMLELEMENT("prod2",
XMLELEMENT("quest",
XMLELEMENT("request",
XMLELEMENT("OutputFormat", 'XML'),
XMLELEMENT("lang", 'ru'),
XMLELEMENT("RequestReq",
XMLELEMENT("User", 'Мyya1'),
XMLELEMENT("Password",'a*1*c*3')
)
)
)
)
),
version '1.0" encoding="windows-1251')),'a*1*c*3',null) as XML from dual;