[FIX] Polynomial Classification: work on sparse

[jerneju]

Issue

Does not work on sparse.

Description of changes

Similar as #45 .

Also fix check for None.

Includes

• [X] Code changes
• [x] Tests
• [ ] Documentation

[codecov-io]

Codecov Report

Merging #52 into master will increase coverage by 3.57%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #52      +/-   ##
=========================================
+ Coverage   91.13%   94.7%   +3.57%     
=========================================
  Files          14      14              
  Lines        1793    1794       +1     
  Branches      216     216              
=========================================
+ Hits         1634    1699      +65     
+ Misses        124      69      -55     
+ Partials       35      26       -9

Impacted Files	Coverage Δ
.../educational/widgets/owpolynomialclassification.py	98.32% <100%> (ø)	:arrow_up:
...gecontrib/educational/widgets/owgradientdescent.py	97.02% <0%> (+15.84%)	:arrow_up:

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[jerneju]

https://sentry.io/biolab/orange3-educational/issues/439995362/

[jerneju]

https://sentry.io/biolab/orange3-educational/issues/440014395/

[PrimozGodec]

Thank you @jerneju. Just noticed today. Works fine.

[jerneju]

👍