constraints added to individual variables don't print when printing the problem

[ericphanson]

e.g.

julia> x = Variable(1000)
Variable
size: (1000, 1)
sign: real
vexity: affine
id: 715…359

julia> for i in 1:1000
       add_constraint!(x, x[i] >= 0)
       end

julia> p = minimize(sum(x))
minimize
└─ sum (affine; real)
   └─ 1000-element real variable (id: 715…359)

status: `solve!` not called yet