Fix deterministic for affine expr with no terms

[odow]

cc @haoxiangyang89

This fixes the "K not defined." But we still throw on:

ERROR: Unable to formulate deterministic equivalent: `copy_and_replace_variables` is not implemented for functions like `-d[1]*G[1] + 0.5 d[1]*s[1,1] + t[0]_out - t[1]_out + 2.5 x[1,1]_out`.

because I didn't address quadratics yet.

[codecov]

Codecov Report

Merging #297 into master will decrease coverage by 0.26%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   93.82%   93.56%   -0.27%     
==========================================
  Files          20       20              
  Lines        1685     1693       +8     
==========================================
+ Hits         1581     1584       +3     
- Misses        104      109       +5

Impacted Files	Coverage Δ
src/deterministic_equivalent.jl	94.28% <50%> (-4.69%)	⬇️
src/algorithm.jl	97.78% <0%> (ø)	⬆️

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[odow]

Something weird is going on with coverage at the moment, because every line is hit, but it's marking the line below the hit line as missing... Merging even with the red x's.