Convert quadratic to affine even when there is no parameters

[blegat]

I am playing around using POI with SumOfSquares and was hitting an error for f - g * p if p is a parameter and g has less monomials than f. Indeed, this will create scalar quadratic constraint with no quadratic term and with no parameter which currently fails.
This PR should fix it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.80%. Comparing base (42d5d6b) to head (8f973c9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   95.39%   95.80%   +0.41%     
==========================================
  Files           5        5              
  Lines        1085     1097      +12     
==========================================
+ Hits         1035     1051      +16     
+ Misses         50       46       -4     

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