[OVAL/probes] handle xsi:nil value in oval_value_to_sexp
[OVAL/probes] handle xsi:nil value in oval_value_to_sexp

redhatrises/openscap,isimluk/openscap,redhatrises/openscap,jan-cerny/openscap,Hexadorsimal/openscap,Hexadorsimal/openscap,OpenSCAP/openscap,jan-cerny/openscap,ybznek/openscap,openprivacy/openscap,mpreisler/openscap,jan-cerny/openscap,openprivacy/openscap,isimluk/openscap,postfix/openscap,postfix/openscap,postfix/openscap,redhatrises/openscap,mpreisler/openscap,mpreisler/openscap,isimluk/openscap,jan-cerny/openscap,openprivacy/openscap,openprivacy/openscap,OpenSCAP/openscap,jan-cerny/openscap,openprivacy/openscap,OpenSCAP/openscap,postfix/openscap,ybznek/openscap,mpreisler/openscap,redhatrises/openscap,ybznek/openscap,isimluk/openscap,mpreisler/openscap,jan-cerny/openscap,postfix/openscap,ybznek/openscap,postfix/openscap,Hexadorsimal/openscap,isimluk/openscap,mpreisler/openscap,redhatrises/openscap,redhatrises/openscap,Hexadorsimal/openscap,openprivacy/openscap,OpenSCAP/openscap,isimluk/openscap,ybznek/openscap,OpenSCAP/openscap,ybznek/openscap,Hexadorsimal/openscap,Hexadorsimal/openscap,OpenSCAP/openscap