Correctly skip land points in the stress update

[einola]

Correctly skip land points in the stress update

Related to #883

Task List

• Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
• Implemented the source code change that satisfies the tests
• Documented the feature by providing worked example
• Updated the README or other documentation
• Completed the pre-Request checklist below

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Change Description

The previous version skipped ocean points and not land points. So I added a not (!) to the commit in PR #884.

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Test Description

The instabilities in #883 are back, but the polynya test case (#797) now runs correctly again.

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Documentation Impact

None

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Other Details

None

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Pre-Request Checklist

• The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
• No new warnings are generated
• The documentation has been updated (or an issue has been created to track the corresponding change)
• Methods and Tests are commented such that they can be understood without having to obtain additional context
• This PR/Issue is labelled as a bug/feature/enhancement/breaking change
• File dates have been updated to reflect modification date
• This change conforms to the conventions described in the README