[BUG] the nav command crashes when run without arguments

[lunkwill42]

Describe the bug

The nav command line program crashes with an AttributeError if run without arguments.

It's not clear when this started happening, but the first observation I've noted goes back to March of 2022.

To Reproduce

1. SSH to a NAV server (or start a NAV development container and enter it using docker exec)
2. Run the nav command
3. See error

Expected behavior

IIRC, the original behavior is just to print the help screen, as the nav command itself does nothing unless a sub-command is specified.

Tracebacks

Traceback (most recent call last):
  File "/usr/sbin/nav", line 342, in <module>
    main(sys.argv[1:])
  File "/usr/sbin/nav", line 53, in main
    args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'

Environment (please complete the following information):

• NAV version installed: 5.6.1

[johannaengland]

Closed by #2603