Correct handling of empty/undef in list values

- Expansion of a list including `None` values, violating 3.2.1 as the
  undefined value is not ignored, e.g.

  >>> expand('/test{/x*}', x=['one', None, 'three'])
  '/test/one/None/three'

- A list with empty string was ignored, violating 2.3 as this should not
  be tread as undefined, example:

  >>> expand('/test{/x}', x=[])
  '/test'
  >>> expand('/test{/x}', x='')
  '/test/'
  >>> expand('/test{/x}', x=[''])
  '/test'  # should be /test/
  >>> expand('/test{/x}', x=['', ''])
  '/test/,'
  >>> expand('/test{/x*}', x=['', ''])
  '/test//'
  >>> expand('/test{/x*}', x=[''])
  '/test'

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