logging: log_cmds: Enable log commands for shell with the logging frontend

When shell is integrated with the logging frontend then pointer to
the logging backend in the shell structure is null. When null pointer
is found assume that it is a shell instance with logging frontend
integration and use frontend in the logging commands.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>