[DOP-22401] Small fixes and optimizations

[dolfinus]

Change Summary

• Do not call dto.merge(new_dto) if dto and new_dto are the same object. This is small optimization, used mostly for locations and datasets, as they are reused as nested fields for other DTOs (inputs, outputs and so on).
• Simplify s bit logic for extract_dataset_and_symlinks - do not call extract_dataset multiple times for the same input.
• SQLAlchemy shows warning about unknown property server_defaults, it should be server_default

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.68%. Comparing base (eadb990) to head (b69a036).
Report is 141 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #150      +/-   ##
===========================================
+ Coverage    92.66%   92.68%   +0.01%     
===========================================
  Files          177      177              
  Lines         3874     3880       +6     
  Branches       267      268       +1     
===========================================
+ Hits          3590     3596       +6     
  Misses         222      222              
  Partials        62       62              

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