Fix is_empty when error thrown while adding constraint

[odow]

Closes #370

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.40%. Comparing base (135f37e) to head (edadfff).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   94.29%   94.40%   +0.11%     
==========================================
  Files           3        3              
  Lines        1051     1055       +4     
==========================================
+ Hits          991      996       +5     
+ Misses         60       59       -1     

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