feat: leave previous answers intact when skipping question

[stereobutter]

With this PR skipping questions using a falsy when doesn't modify the answers file anymore. This enables the usage of when: "{{answer is not defined}}" to only ask a question on copier copy and skip the question on any future copier update while keeping the original answer recorded.

# copier.yaml
year_of_first_publication:
    type: str
    when: "{{year_of_first_publication is not defined}}"

resolves #1574

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.30%. Comparing base (eeeebc3) to head (1118ba2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1582   +/-   ##
=======================================
  Coverage   97.29%   97.30%           
=======================================
  Files          48       48           
  Lines        4621     4636   +15     
=======================================
+ Hits         4496     4511   +15     
  Misses        125      125           

Flag	Coverage Δ
unittests	97.30% <100.00%> (+<0.01%)	⬆️

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[stereobutter]

Oh and I also tried to make the docs for when a little bit clearer with regards to what skipping a question actually means (under the new behaviour this PR introduces).

[sisp]

Thanks for your initiative, @stereobutter! 🙏 Unfortunately, I don't think this idea works – see #1574 (comment). Let's keep the discussion there for now until we have a working design proposal.

[yajo]

Thanks for your efforts @stereobutter! 😊 I'll close this pr to keep our list clean, because there have been hopefully better solutions proposed in the issue. Let's continue there until we reach consensus.