Fixed Python 3 bug in clip visitor.
Fixed Python 3 bug in clip visitor.

The clip visitor assumed that `None` could be compared to a string,
which was true for Python 2 but is no longer true for Python 3. For
example, `None < 'hello'` is `True` for Python 2 but raises a
`TypeError` for Python 3.
HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper,HaroldMills/Vesper