Default window frames to not match PostgreSQL about datafusion-python

[timsaucer]

This seems like a very easy fix. I think we just match on order_by.is_some() and pick the appropriate default. To close, I think at a minimum we would want (1) a unit test to cover both options and (2) text in the window function that describes this behavior.

[timsaucer]

Based on recommendation on discord, we may want to use WindowFrame::new() which has the logic already for checking if order_by exists.

[timsaucer]

From @Michael-J-Ward

Doing a little archaelogy on that:

This is the PR where window_frame switched from None to WindowFrame::new(order_by.is_some());

The PR discussion has some talk on test regressions related to it.

https://github.com/apache/datafusion-python/pull/115/files#diff-676f1190ad1b28e57e9eac75de61cccc59420615e3da10f91a493fb48d0711ff