BUG - does not work when NCS is not specified in west.yml

[TjazVracko]

Describe the bug east update toolchain explodes with the following error when using a west.yml file with no NCS (aka bare ephyr). west.yml is attached west.yml.txt

To Reproduce

λ east update toolchain
Traceback (most recent call last):
  File "/home/tjaz/anaconda3/bin/east", line 8, in <module>
    sys.exit(main())
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/east/__main__.py", line 73, in main
    cli()
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/rich_click/rich_group.py", line 21, in main
    rv = super().main(*args, standalone_mode=False, **kwargs)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/core.py", line 1654, in invoke
    super().invoke(ctx)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/east/__main__.py", line 59, in cli
    ctx.obj = EastContext(echo)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/east/east_context.py", line 71, in __init__
    self.detected_ncs_version = get_ncs_version(self.west_dir_path)
  File "/home/tjaz/anaconda3/lib/python3.8/site-packages/east/helper_functions.py", line 183, in get_ncs_version
    return ncs[0]["revision"]
IndexError: list index out of range

λ east --version
0.3.0