`project`ing to a nonexistent address gives 0.0 (and doesn't complain)

[bzinberg]

I think Gen should complain in this case, because otherwise typos can silently lead to unintended behavior:

julia> @gen function f()
         {:address} ~ Gen.bernoulli(0.5)
       end;

julia> t = Gen.simulate(f, ());

julia> Gen.project(t, Gen.select(:address))
-0.6931471805599453

julia> Gen.project(t, Gen.select(:addres))
0.0