BUG: Fix preceding and following with None

[cpcloud]

Before this PR, calls like

ibis.window(preceding=(None, 1))  # ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
ibis.window(following=(1, None))  # ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING

would fail because of incorrect usage of comparison operators, comparing
None to another value that is not None raising a TypeError.

This PR addresses the problem.

[cpcloud]

Thanks for the review @ash32 I ended up doing it slightly different, but the end result was the same (checking the value of start/end regardless of whether its counterpart was None).