support kinetic laws with only cn elements

[codecov[bot]]

Codecov Report

Merging #131 (fe2a939) into main (76c91e9) will increase coverage by 0.84%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   93.94%   94.79%   +0.84%     
==========================================
  Files           7        7              
  Lines         347      346       -1     
==========================================
+ Hits          326      328       +2     
+ Misses         21       18       -3     

Impacted Files	Coverage Δ
src/reactions.jl	97.50% <100.00%> (+0.04%)	:arrow_up:

... and 2 files with indirect coverage changes

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