Fix xr_combine_kwargs in extract_dataset

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• CHANGES.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• In Fixes for xclim 0.50 - Better defaults for opening #398, I introduced a new way of setting defaults for xr_combine_kwargs in extract_dataset, but I missed one use of the argument, which make extract_dataset fail when nothing is given to the argument.

Does this PR introduce a breaking change?

No.

Other information:

Seen by Dominic et Oumou.