Fix empty sums in Aff/QuadExpr parse_expression

[odow]

Closes #3090

Now produces

julia> model = Model();

julia> @variable(model, x);

julia> @expression(model, expr, x^2);

julia> @NLconstraint(model, expr >= 3)
x * x - 3.0 ≥ 0

[codecov]

Codecov Report

Base: 97.62% // Head: 97.63% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ae34a75) compared to base (4e276d9).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3091   +/-   ##
=======================================
  Coverage   97.62%   97.63%           
=======================================
  Files          32       32           
  Lines        4303     4318   +15     
=======================================
+ Hits         4201     4216   +15     
  Misses        102      102           

Impacted Files	Coverage Δ
src/nlp.jl	100.00% <100.00%> (ø)

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