Add test for empty matrix tr_mul.

tests/core/empty.rs

@@ -1,9 +1,30 @@
 use na::{DMatrix, DVector};
 
 #[test]
-fn empty_mul() {
+fn empty_matrix_mul_vector() {
     // Issue #644
     let m = DMatrix::<f32>::zeros(8, 0);
     let v = DVector::<f32>::zeros(0);
     assert_eq!(m * v, DVector::zeros(8));
+}
+
+#[test]
+fn empty_matrix_mul_matrix() {
+    let m1 = DMatrix::<f32>::zeros(3, 0);
+    let m2 = DMatrix::<f32>::zeros(0, 4);
+    assert_eq!(m1 * m2, DMatrix::zeros(3, 4));
+}
+
+#[test]
+fn empty_matrix_tr_mul_vector() {
+    let m = DMatrix::<f32>::zeros(0, 5);
+    let v = DVector::<f32>::zeros(0);
+    assert_eq!(m.tr_mul(&v), DVector::zeros(5));
+}
+
+#[test]
+fn empty_matrix_tr_mul_matrix() {
+    let m1 = DMatrix::<f32>::zeros(0, 3);
+    let m2 = DMatrix::<f32>::zeros(0, 4);
+    assert_eq!(m1.tr_mul(&m2), DMatrix::zeros(3, 4));
 }