Fix canonicalize! for empty

[blegat]

Closes #24

[coveralls]

Coverage increased (+0.02%) to 86.123% when pulling 29cecc1 on bl/emptycanon into b188b48 on master.

[coveralls]

Coverage increased (+0.02%) to 86.123% when pulling 29cecc1 on bl/emptycanon into b188b48 on master.

[codecov]

Codecov Report

Merging #25 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage    86.1%   86.12%   +0.01%     
==========================================
  Files          10       10              
  Lines         698      699       +1     
==========================================
+ Hits          601      602       +1     
  Misses         97       97

Impacted Files	Coverage Δ
src/operations.jl	89.26% <100%> (+0.07%)	⬆️

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