fix: writexml handles missing parameter configs for normfactor

[kratsg]

Pull Request Description

Fixes #1814.

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• Tests are passing
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* In situations in writexml where the normfactor parameter config is missing
inits or bounds, fall back to using default values instead.
* Add tests that use workspaces with no parameter inits and no parameter bounds
to cover this behavior.

[codecov]

Codecov Report

Merging #1819 (34f37dd) into master (36e0d67) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1819   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files          68       68           
  Lines        4310     4310           
  Branches      725      725           
=======================================
  Hits         4231     4231           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.42% <0.00%> (ø)
doctest	60.74% <0.00%> (ø)
unittests-3.10	96.03% <100.00%> (ø)
unittests-3.7	96.01% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/pyhf/writexml.py	95.83% <100.00%> (ø)

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[matthewfeickert]

Thanks for the quick fix @kratsg. LGTM.