`wp plugin update` runs with missing argument, 

`wp plugin update` accepts calls without `<plugin>` or `--all`, makes no updates, and succeeds. ;)

Can we print a warning message instead?

    wp plugin update
    Success: Updated 0/0 plugins.
    # oops, I meant --all
    wp plugin update --all
    Enabling Maintenance mode...
    # …
    Success: Updated 3/3 plugins.