Fix nullptr dereference issue in triangular product.

Eigen/src/Core/products/TriangularMatrixMatrix.h

diff --git a/Eigen/src/Core/products/TriangularMatrixMatrix.h b/Eigen/src/Core/products/TriangularMatrixMatrix.h
index 770107a..94eabdc 100644
--- a/Eigen/src/Core/products/TriangularMatrixMatrix.h
+++ b/Eigen/src/Core/products/TriangularMatrixMatrix.h
@@ -422,6 +422,12 @@
     internal::add_const_on_value_type_t<ActualLhsType> lhs = LhsBlasTraits::extract(a_lhs);
     internal::add_const_on_value_type_t<ActualRhsType> rhs = RhsBlasTraits::extract(a_rhs);
 
+    // Empty product, return early.  Otherwise, we get `nullptr` use errors below when we try to access
+    // coeffRef(0,0).
+    if (a_lhs.size() == 0 || a_rhs.size() == 0) {
+      return;
+    }
+
     LhsScalar lhs_alpha = LhsBlasTraits::extractScalarFactor(a_lhs);
     RhsScalar rhs_alpha = RhsBlasTraits::extractScalarFactor(a_rhs);
     Scalar actualAlpha = alpha * lhs_alpha * rhs_alpha;