[BUG] SoA request is ignored with external c++

It looks like stanc ignores the -fsoa flag if any undefined functions are used in the likelihood. Given the following model:

functions {
  real undefined_fun(vector x);
}
data {
  int Npars;
}

parameters {
  vector[Npars] pars;
}

model {
  target += undefined_fun(pars);
}

I get the following generated c++:

...
Eigen::Matrix<local_scalar_t__,-1,1> pars =
  Eigen::Matrix<local_scalar_t__,-1,1>::Constant(Npars, DUMMY_VAR__);
...

If I remove the undefined_fun() call from the likelihood:

functions {
  real undefined_fun(vector x);
}
data {
  int Npars;
}

parameters {
  vector[Npars] pars;
}

model {
  target += sum(pars);
}

I instead get:

...
stan::math::var_value<Eigen::Matrix<double,-1,1>> pars =
  stan::math::var_value<Eigen::Matrix<double,-1,1>>(Eigen::Matrix<double,-1,1>::Constant(Npars,
                                                      std::numeric_limits<double>::quiet_NaN(
                                                        )));
...

Both models were called with ./mac-stanc ./soa_test.stan -fsoa --allow-undefined using the Oct 23 nightly (the latest release)

Reproduce the issue with the supplied Stan models using ./mac-stanc ./soa_test.stan -fsoa --allow-undefined. Compare the generated C++ when undefined_fun is used with the output for sum(pars); done means the -fsoa request is honored even when external C++ functions are undefined.