Fix Traceback when rolling back session

[soininen]

This fixes a Traceback when calling rollback_session() after purging a database and adding items with same unique keys as the purged ones.

No associated issue.

Checklist before merging

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

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.68%. Comparing base (bc58e16) to head (4671b99).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/db_mapping_base.py	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
+ Coverage   85.66%   85.68%   +0.02%     
==========================================
  Files          81       81              
  Lines       11029    11034       +5     
  Branches     1623     1623              
==========================================
+ Hits         9448     9455       +7     
+ Misses       1224     1223       -1     
+ Partials      357      356       -1     

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