Fix problem when importing values from SpineDBManager

[soininen]

Manager's cache is slightly different than what we're used to in spinedb_api. We should still accept it.

Fixes Spine-project/Spine-Toolbox#1851

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