feat: Add POI-less specification support

[kratsg]

Pull Request Description

See #1602 and #1636 (and of course #950).

A specification containing a measurement with "poi": "" should be treated as POI-less. Do not currently support "poi": null at the moment, but this could be considered later if there is desire for it - however the choice of empty string is primarily to align with the XML schema.

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• 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

* Support JSON specifications with empty string for POI by treating as POI-less
* Support poi_name as empty string, in addition to None
* Add test for POI-less workspace

[codecov]

Codecov Report

Merging #1638 (3670949) into master (ae92c25) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1638   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files          63       63           
  Lines        4128     4129    +1     
  Branches      566      566           
=======================================
+ Hits         4047     4048    +1     
  Misses         48       48           
  Partials       33       33           

Flag	Coverage Δ
contrib	24.99% <0.00%> (-0.01%)	⬇️
unittests	97.82% <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	97.70% <100.00%> (+<0.01%)	⬆️

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[alexander-held]

@Daniel-Noel this might be interesting for you (and maybe the already merged #1636 even more so).

[matthewfeickert]

LGTM. 👍 Thanks!