feat: Add support for _ModelConfig.set_poi(None) to unset model POI

[kratsg]

Pull Request Description

Resolves #1984 by allowing _ModelConfig.set_poi(None) to be allowed behavior to remove the POI from a model.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Allow for `_ModelConfig.set_poi(None)` as a way to unset the POI from a model.
* Add test of `model.config.set_poi` for value of the poi that are valid strings and None.

[codecov]

Codecov Report

Base: 98.25% // Head: 98.25% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (405a51c) compared to base (e4aae10).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1985   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files          68       68           
  Lines        4416     4420    +4     
  Branches      726      727    +1     
=======================================
+ Hits         4339     4343    +4     
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.94% <0.00%> (-0.03%)	⬇️
doctest	60.83% <0.00%> (-0.06%)	⬇️
unittests-3.10	96.17% <100.00%> (+<0.01%)	⬆️
unittests-3.7	96.15% <100.00%> (+<0.01%)	⬆️
unittests-3.8	96.19% <100.00%> (+<0.01%)	⬆️
unittests-3.9	96.22% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/pyhf/pdf.py	98.01% <100.00%> (+0.02%)	⬆️

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[matthewfeickert]

LGTM. 👍 Thanks for being so fast on this @kratsg and for the quick iteration @alexander-held.