Only replace mhchem arrows when mhchem versions are available. (mathjax/MathJax#3452)

[dpvc]

This PR fixes a problem in mhchem where the replacement of arrow macros with mhchem-spoecific ones (in order to use the mhchem font extension) is overly aggressive and replaces some macros for there there is no mhchem equivalent. The change makes the replace() call check that an mhchem version exists before replacing the arrow macro.

Resolves issue mathjax/MathJax#3452.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.66%. Comparing base (27c47f3) to head (dec9d2a).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1372     +/-   ##
==========================================
  Coverage    86.66%   86.66%             
==========================================
  Files          338      338             
  Lines        84384    84396     +12     
  Branches      4779     3152   -1627     
==========================================
+ Hits         73131    73143     +12     
  Misses       11253    11253             

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