util: json: Add option to skip adding a JSON object if it is NULL
util: json: Add option to skip adding a JSON object if it is NULL

Add a new option specifier that will optionally add a JSON key=value pair
containing a nested object if the added object isn't NULL.

jardasgit/libvirt,agx/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,taget/libvirt,rlaager/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,olafhering/libvirt,VenkatDatta/libvirt,datto/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,rlaager/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,taget/libvirt,crobinso/libvirt,andreabolognani/libvirt,shugaoye/libvirt,shugaoye/libvirt,jfehlig/libvirt,nertpinx/libvirt,elmarco/libvirt,shugaoye/libvirt,jardasgit/libvirt,eskultety/libvirt,jfehlig/libvirt,eskultety/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,datto/libvirt,agx/libvirt,rlaager/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,VenkatDatta/libvirt,crobinso/libvirt,VenkatDatta/libvirt,olafhering/libvirt,agx/libvirt,eskultety/libvirt,agx/libvirt,elmarco/libvirt,elmarco/libvirt,fabianfreyer/libvirt,taget/libvirt,eskultety/libvirt,elmarco/libvirt,nertpinx/libvirt,datto/libvirt,fabianfreyer/libvirt,elmarco/libvirt,rlaager/libvirt,agx/libvirt,shugaoye/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,taget/libvirt,zippy2/libvirt,taget/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,rlaager/libvirt,eskultety/libvirt,crobinso/libvirt,datto/libvirt,jfehlig/libvirt,nertpinx/libvirt