fix: Handle importing XML measurements with no POI attribute

[kratsg]

Description

See #1909.

Add runtime check for missing POI in XML measurements. XML measurements are not necessarily required to have a POI, but we require it, so need to fix coverage.

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

* Add runtime check for missing POI in XML measurements. XML measurements are not
necessarily required to have a POI, but we require it, so need to fix coverage.
* Add test and test XML files for missing POI in XML configs.

[codecov]

Codecov Report

Merging #1933 (1915293) into master (18bc4ec) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1933   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files          68       68           
  Lines        4330     4334    +4     
  Branches      729      730    +1     
=======================================
+ Hits         4253     4257    +4     
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.34% <0.00%> (-0.03%)	⬇️
doctest	60.61% <0.00%> (-0.06%)	⬇️
unittests-3.10	96.10% <100.00%> (+<0.01%)	⬆️
unittests-3.7	96.08% <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/readxml.py	95.88% <100.00%> (+0.09%)	⬆️

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[matthewfeickert]

LGTM. 👍 Thanks @kratsg!