Fix opening DB editor tab when editor window has a empty tab only

[soininen]

No associated issue

Checklist before merging

• Documentation 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

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.13%. Comparing base (554ef57) to head (d2275e9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/ui_main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3003      +/-   ##
==========================================
+ Coverage   67.12%   67.13%   +0.01%     
==========================================
  Files         193      193              
  Lines       37102    37102              
  Branches     4445     4445              
==========================================
+ Hits        24904    24910       +6     
- Misses      10932    10935       +3     
+ Partials     1266     1257       -9     

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