Stop iterating import mappings when row gets skipped

[soininen]

This PR fixes a bug where import_mappings would produce repeated mapped data in case there were rows that should have been skipped.

Fixes spine-tools/Spine-Toolbox#2749

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.19%. Comparing base (7a2db7f) to head (5283a0a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ========================================== - Coverage 78.21% 78.19% -0.02% ========================================== Files 76 76 Lines 9509 9510 +1 Branches 2016 2016 ========================================== - Hits 7437 7436 -1 - Misses 1737 1738 +1 - Partials 335 336 +1 ```

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