Fix Traceback in get_parameter_value_item()

[soininen]

This PR fixes DatabaseMapping.get_parameter_value_item() which would Traceback when the parameter definition used a value list but the parameter value did not exists, e.g. when it is not defined in a specific alternative.

Fixes #401

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

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.23%. Comparing base (5be24ac) to head (2505b0c).

Files	Patch %	Lines
spinedb_api/mapped_items.py	66.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #402 +/- ## ========================================== - Coverage 78.24% 78.23% -0.01% ========================================== Files 76 76 Lines 9501 9506 +5 Branches 2013 2014 +1 ========================================== + Hits 7434 7437 +3 - Misses 1734 1735 +1 - Partials 333 334 +1 ```

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