removed redundant if in determinant

Author: t4c1

stan/math/prim/fun/determinant.hpp

@@ -20,10 +20,6 @@ namespace math {
 template <typename T, require_eigen_vt<std::is_arithmetic, T>* = nullptr>
 inline value_type_t<T> determinant(const T& m) {
   check_square("determinant", "m", m);
-  if (m.size() == 0) {
-    return 1;
-  }
-
   return m.determinant();
 }