consistent trace_gen_inv_quad_form_ldlt behavior at size 0 #1447

Description

The following test should pass in test/unit/math/mix/mat/fun/trace_gen_inv_quad_form_ldlt_test.cpp.

  Eigen::MatrixXd a00(0, 0);
  Eigen::MatrixXd b00(0, 0);
  Eigen::MatrixXd c00(0, 0);
  stan::test::expect_ad(f, c00, a00, b00);

As is, the boundary behavior doesn't match among the primitive and autodiff versions of the function.

Current Version:

v3.0.0