fix: Make default sort order nulls last

[nuno-faria]

Which issue does this PR close?

Closes #1765.

Rationale for this change

Make sort follow the same default as upstream (i.e., order nulls last).

What changes are included in this PR?

• Changed the default value of nulls_first to False.
• Added a unit test / updated previous tests.

Are there any user-facing changes?

df.sort without an explicit nulls_first orders nulls last, similarly to df.sort_by and SQL's ORDER BY.

[timsaucer]

Thanks! This is changing a public facing API so I think we need an entry in the upgrade guide. It's small, but it's important.

[nuno-faria]

Thanks! This is changing a public facing API so I think we need an entry in the upgrade guide. It's small, but it's important.

Added. Btw I think the upgrade guide in the website is not being updated, 55.0.0 is currently missing: https://datafusion.apache.org/python/user-guide/upgrade-guides.html

[timsaucer]

Thanks! This is changing a public facing API so I think we need an entry in the upgrade guide. It's small, but it's important.

Added. Btw I think the upgrade guide in the website is not being updated, 55.0.0 is currently missing: https://datafusion.apache.org/python/user-guide/upgrade-guides.html

I think the website only updates when we do the release. You can see them on the staged site which comes off main: https://datafusion.staged.apache.org/python/user-guide/upgrade-guides.html