fix: don't share default values in ArchiveField

[giovanni-guidini]

Passing an object as the default value to ArchiveField (such as {})
means that we always return the same reference to multiple objects. So they're shared.
(thanks @scott-codecov for pointing that out)

So instead we will pass a class and call the class, creating a new object if we have to use it.
Noticed I hacked it a little for the default of the defaults to return None.
We can do the same if we ever need more complex values, or just create a new class.

I also moved the log of dbfield and archive_field None to be DEBUG and not INFO.
We are getting that quite a lot and it's making support's work hard.

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[codecov]

Codecov Report

Merging #67 (bb82834) into main (ad044e4) will not change coverage.
The diff coverage is 100.00%.

Changes have been made to critical files, which contain lines commonly executed in production. Learn more

@@           Coverage Diff           @@
##             main      #67   +/-   ##
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  Coverage   98.59%   98.59%           
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  Files         357      357           
  Lines       26004    26004           
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  Hits        25639    25639           
  Misses        365      365           

Flag	Coverage Δ
integration	98.56% <100.00%> (ø)
latest-uploader-overall	98.56% <100.00%> (ø)
onlysomelabels	98.59% <100.00%> (ø)
unit	98.56% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
NonTestCode	97.32% <100.00%> (ø)
OutsideTasks	98.33% <100.00%> (ø)

Files Changed	Coverage Δ
database/models/core.py	96.92% <ø> (ø)
database/models/reports.py	99.19% <ø> (ø)
database/tests/unit/test_model_utils.py	100.00% <100.00%> (ø)
database/utils.py Critical	96.96% <100.00%> (ø)

Related Entrypoints
run/app.tasks.upload.UploadProcessor
run/app.tasks.notify.Notify
run/app.tasks.upload.Upload
run/app.tasks.upload.UploadFinisher
run/app.tasks.pulls.Sync
run/app.tasks.compute_comparison.ComputeComparison
run/app.tasks.timeseries.backfill_commits