deep_copy_value() to accept None

[soininen]

None is a valid parameter value.

No associated issue.

Checklist before merging

• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.32%. Comparing base (0cc12df) to head (a6fb829).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #383 +/- ## ======================================== Coverage 78.32% 78.32% ======================================== Files 75 75 Lines 9504 9504 Branches 2015 2015 ======================================== Hits 7444 7444 Misses 1727 1727 Partials 333 333 ```

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