Fixing missing=0 in convert calendar

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Convert_calendar bugs with falsy missing value. #1562, fixes https://github.com/Ouranosinc/ESPO-Internal/issues/3
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• 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?

• When missing=0 is passed to convert_calendar, it is used instead of replaced by np.NaN by faulty expression.

Does this PR introduce a breaking change?

No.

Other information:

Xarray's implementation does not have this problem.