Fix storing/loading database references for Data connections

[soininen]

This PR fixes a bug where database credentials were always stored under "None" instead of sanitized database URL.

Re spine-tools/Spine-Toolbox#2329

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 (37f0ab2) 60.22% compared to head (22e6870) 60.27%.

Files	Patch %	Lines
spine_items/utils.py	93.54%	1 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #175 +/- ## =========================================== + Coverage 60.22% 60.27% +0.04% =========================================== Files 194 194 Lines 17678 17682 +4 Branches 2886 2887 +1 =========================================== + Hits 10647 10658 +11 + Misses 6462 6457 -5 + Partials 569 567 -2 ```

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