Fix lmod.savelist return when there is no collections
Fix lmod.savelist return when there is no collections

If there are no collections, module savelist returns
the message "No named collections." in stdout and stderr
is left empty. In this case, the lmod function return
value is not a string but None, which was not considered
in the previous version of the function.

cmd-ntrf/jupyter-lmod,cmd-ntrf/jupyter-lmod,cmd-ntrf/jupyter-lmod