Fix a bug where nD entity classes failed to import

[soininen]

This PR fixes a corner case where empty row in an input table would cause the dimensions of an entity class to become duplicated.

Fixes spine-tools/Spine-Toolbox#2760

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

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.22%. Comparing base (fc4b515) to head (e2aea79).

Files	Patch %	Lines
spinedb_api/import_mapping/import_mapping.py	66.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #404 +/- ## ========================================== - Coverage 78.23% 78.22% -0.02% ========================================== Files 76 76 Lines 9506 9509 +3 Branches 2014 2016 +2 ========================================== + Hits 7437 7438 +1 - Misses 1735 1736 +1 - Partials 334 335 +1 ```

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