fix: Coverage for importing XML channels with no data

[kratsg]

Description

Handle cases where channel is missing data in the XML configuration (associated with Asimov). c.f. Issue #1911

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

* Raise RuntimeError for cases where channel is missing data in the XML configuration
(associated with Asimov).
   - c.f. Issue #1911
* Add tests and test XML files.

[codecov]

Codecov Report

Merging #1931 (f150460) into master (1494b66) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1931      +/-   ##
==========================================
+ Coverage   98.17%   98.22%   +0.04%     
==========================================
  Files          68       68              
  Lines        4330     4330              
  Branches      729      729              
==========================================
+ Hits         4251     4253       +2     
+ Misses         46       45       -1     
+ Partials       33       32       -1     

Flag	Coverage Δ
contrib	26.37% <0.00%> (ø)
doctest	60.66% <0.00%> (ø)
unittests-3.10	96.09% <100.00%> (+0.04%)	⬆️
unittests-3.7	96.08% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
src/pyhf/readxml.py	95.78% <100.00%> (+1.20%)	⬆️

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[matthewfeickert]

LGTM. 👍 Thanks @kratsg!