Allow cleanAttributes to work on nodes with no attributes (e.g., XML nodes)

[dpvc]

This PR allows the TeX input jax's cleanAttributes() filter to work with nodes that don't have attributes (like XML nodes). Currently, it will crash with an error about not being able to call get() on an undefined value.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.66%. Comparing base (fe4921f) to head (8505432).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1368   +/-   ##
========================================
  Coverage    86.66%   86.66%           
========================================
  Files          338      338           
  Lines        84269    84270    +1     
  Branches      4784     4785    +1     
========================================
+ Hits         73032    73033    +1     
  Misses       11214    11214           
  Partials        23       23           

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[zorkow]

lgtm.