Offer to authenticate if there are no workspaces

[rusq]

When running slackdump for the first time, i.e.

./slackdump archive

without prior authentication, it would output the cryptic error message:

$ slackdump archive
archive: 2024/07/27 07:24:52 Error 004 (Authentication Error): auth error: workspace does not exist: ""
Error 004 (Authentication Error): auth error: workspace does not exist: ""
exit status 4

This PR introduces the change that handles this case correctly, offering to authenticate in a new workspace.