Fix changing Importer's mapping type

[soininen]

Importer was using legacy functionality in spinedb_api to get blank mappings when user changed the mapping type. This did not work e.g. with Entity group mappings since those did not exist in pre-0.7 world.

Fixes spine-tools/Spine-Toolbox#2662 spine-tools/Spine-Toolbox# (issue)

Checklist before merging

• [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 61.68%. Comparing base (7cf1886) to head (623f2fb). Report is 1 commits behind head on 0.8-dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #197 +/- ## =========================================== + Coverage 61.63% 61.68% +0.05% =========================================== Files 190 190 Lines 17656 17656 Branches 2890 2890 =========================================== + Hits 10882 10891 +9 + Misses 6189 6183 -6 + Partials 585 582 -3 ```

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