Fix pivoted import mappings using column headers

[soininen]

In certain scenarios mapping's value would be accidentally set to None which would change the meaning of mapping's position from "column header" to "headers".

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

All modified and coverable lines are covered by tests ✅

Project coverage is 80.27%. Comparing base (ff79412) to head (f2d831f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   80.24%   80.27%   +0.03%     
==========================================
  Files          76       76              
  Lines        9970     9970              
  Branches     1475     1475              
==========================================
+ Hits         8000     8003       +3     
+ Misses       1626     1625       -1     
+ Partials      344      342       -2     

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