fix(backport): Test xml.etree.ElementTree.Element truth value by 'is not None'

[meeseeksmachine]

Backport PR #2459: fix: Test xml.etree.ElementTree.Element truth value by 'is not None'

* Backport PR https://github.com/scikit-hep/pyhf/pull/2459
* In Python 3.14 testing the truth value of an xml.etree.ElementTree.Element is
  deprecated and will raise an exception. As of Python 3.12 this behavior will
  raise a DeprecationWarning:

  ```
  DeprecationWarning: Testing an element's truth value will raise an exception
  in future versions.  Use specific 'len(elem)' or 'elem is not None' test
  instead.
  ```

  To avoid this, determine the truth element by using the 'elem is not None'
  method.
   - c.f. https://docs.python.org/3.12/library/xml.etree.elementtree.html#element-objects

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Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

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Files	Patch %	Lines
src/pyhf/writexml.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2499   +/-   ##
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  Coverage                  ?   98.19%           
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  Files                     ?       69           
  Lines                     ?     4545           
  Branches                  ?      803           
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  Hits                      ?     4463           
  Misses                    ?       49           
  Partials                  ?       33           

Flag	Coverage Δ
contrib	97.77% <0.00%> (?)
doctest	98.06% <0.00%> (?)
unittests-3.10	96.23% <0.00%> (?)
unittests-3.11	96.23% <0.00%> (?)
unittests-3.12	96.23% <0.00%> (?)
unittests-3.7	96.21% <0.00%> (?)
unittests-3.8	96.25% <0.00%> (?)
unittests-3.9	96.28% <0.00%> (?)

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