Nan edgecolor scattermap

[sarahclaude]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• [X ] (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• 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?

• Xarray was able to handle NaNs but not when used with edgecolor for scatter map so masks were added

Does this PR introduce a breaking change?

No

Other information:

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[vindelico]

This corrects the issue where edges where plotted where values are NaN.