Fix eigenvalues_sym return type

Author: WardBrian

stan/math/prim/fun/eigenvalues_sym.hpp

@@ -21,8 +21,7 @@ namespace math {
  */
 template <typename EigMat, require_eigen_matrix_dynamic_t<EigMat>* = nullptr,
           require_not_st_var<EigMat>* = nullptr>
-Eigen::Matrix<value_type_t<EigMat>, Eigen::Dynamic, 1> eigenvalues_sym(
-    const EigMat& m) {
+Eigen::Vector<value_type_t<EigMat>, -1> eigenvalues_sym(const EigMat& m) {
   using PlainMat = plain_type_t<EigMat>;
   const PlainMat& m_eval = m;
   check_nonzero_size("eigenvalues_sym", "m", m_eval);

stan/math/rev/fun/eigenvalues_sym.hpp

@@ -23,10 +23,8 @@ namespace math {
  */
 template <typename T, require_rev_matrix_t<T>* = nullptr>
 inline auto eigenvalues_sym(const T& m) {
-  using return_t = return_var_matrix_t<T>;
-  if (unlikely(m.size() == 0)) {
-    return return_t(m);
-  }
+  using return_t = return_var_matrix_t<Eigen::VectorXd, T>;
+  check_nonzero_size("eigenvalues_sym", "m", m);
   check_symmetric("eigenvalues_sym", "m", m);
 
   auto arena_m = to_arena(m);