Use { NULL } instead of NULL as envp when executing remediations
Use { NULL } instead of NULL as envp when executing remediations

Using just NULL is discouraged and nonportable, see `man 2 execve`.
This commit doesn't change behavior, just improves portability.

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