Fix index ordering when merging values on import

[soininen]

In case of string indices, we want to append the new parameter after the old one.

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

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.20%. Comparing base (d076802) to head (152d1d1).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/parameter_value.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #463   +/-   ##
=======================================
  Coverage   79.20%   79.20%           
=======================================
  Files          77       77           
  Lines       10111    10112    +1     
  Branches     1510     1509    -1     
=======================================
+ Hits         8008     8009    +1     
- Misses       1756     1757    +1     
+ Partials      347      346    -1     

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