Fix diagm_container for types without zero

[blegat]

For some types, T, zero(T) isa T does not hold. This is the case for instance for T = JuMP.VariableRef for which zero(T) isa AffExpr. When the users call diagm with vectors of VariableRef, we should have a matrix of AffExpr but instead an error is thrown

MethodError: Cannot `convert` an object of type JuMP.AffExpr to an object of type JuMP.VariableRef
Closest candidates are:
  convert(::Type{T}, !Matched::T) where T at essentials.jl:171

because diagm_container calls zeros(JuMP.VariableRef, ...).
I think it is fair for zeros(JuMP.VariableRef, ...) to error and it would not be consistent for it to return a matrix of JuMP.AffExpr so it seems the issue is that diagm_container calls zeros(JuMP.VariableRef, ...) hence the fix in this PR.

Note that if you try it with the latest version of JuMP, it currently works as we redefine diagm_container just for this case:
https://github.com/JuliaOpt/MutableArithmetics.jl/blob/407a8fdfb5ef1810f3785a88e490291d37b9fb03/src/dispatch.jl#L27-L31
but we'd rather have it fixed upstream as other types might benefit from it (e.g. polynomial variables from MultivariatePolynomials).

See jump-dev/MutableArithmetics.jl#47 for more context.

[blegat]

Bump

[dkarrasch]

Seems reasonable to me.