Fix database migration when database has no multidimensional entity classes

[soininen]

This PR fixes the migration script that add active_by_default column to entity_class tables. The script resulted in a Traceback when the upgradeable database did not have multidimensional entity classes.

Fixes spine-tools/Spine-Toolbox#2512

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-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (782b041) 77.14% compared to head (74d39ad) 77.04%.

Files	Patch %	Lines
...eff478bcb_add_active_by_default_to_entity_class.py	0.00%	1 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #339 +/- ## =========================================== - Coverage 77.14% 77.04% -0.10% =========================================== Files 75 75 Lines 9239 9241 +2 Branches 1949 1950 +1 =========================================== - Hits 7127 7120 -7 - Misses 1780 1786 +6 - Partials 332 335 +3 ```

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