Default constructor to system_functor leaves raw pointer uninitialized (UB if read)

Description:

InSTAN_MATH_REV_MAT_FUNCTOR_ALGEBRA_SYSTEM_HPP, we have:

struct system_functor {
  F f_;
  Eigen::Matrix<T0, Eigen::Dynamic, 1> x_;
  Eigen::Matrix<T1, Eigen::Dynamic, 1> y_;
  std::vector<double> dat_;
  std::vector<int> dat_int_;
  std::ostream* msgs_;

  system_functor() {}

  system_functor(const F& f, const Eigen::Matrix<T0, Eigen::Dynamic, 1>& x,
                 const Eigen::Matrix<T1, Eigen::Dynamic, 1>& y,
                 const std::vector<double>& dat,
                 const std::vector<int>& dat_int, std::ostream* msgs)
      : f_(f), x_(x), y_(y), dat_(dat), dat_int_(dat_int), msgs_(msgs) {}

It seems like the default constructor should initialize msgs_ to nullptr to avoid msgs_ being some random value in the best case? This is UB if the pointer is either read or dereferenced.

Current Version:

v2.17.0

Open STAN_MATH_REV_MAT_FUNCTOR_ALGEBRA_SYSTEM_HPP and inspect the default constructor of system_functor. Ensure msgs_ has a defined null state when that constructor is used, then run the relevant existing test suite or build checks to confirm the change does not break compilation.