fix: handle non-concrete polynomials in `polynomial_merge!`#339

blegat merged 3 commits intoJuliaAlgebra:masterJuliaAlgebra/MultivariatePolynomials.jl:masterfrom

AayushSabharwal:as/poly-merge-nonconcreteAayushSabharwal/MultivariatePolynomials.jl:as/poly-merge-nonconcreteCopy head branch name to clipboard