Add NullLogger class
Add NullLogger class

Implements an empty Logger. Messages are discarded.

Closes: #499
Approved by: m-blaha

hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,hughsie/libhif,Conan-Kudo/libhif,Conan-Kudo/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libhif