pitfall on merge iterator

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm/src/tests/day2.rs

@@ -120,6 +120,7 @@ fn test_task2_merge_1() {
         (Bytes::from("a"), Bytes::from("1.1")),
         (Bytes::from("b"), Bytes::from("2.1")),
         (Bytes::from("c"), Bytes::from("3.1")),
+        (Bytes::from("e"), Bytes::new()),
     ]);
     let i2 = MockIterator::new(vec![
         (Bytes::from("a"), Bytes::from("1.2")),
@@ -146,6 +147,7 @@ fn test_task2_merge_1() {
             (Bytes::from("b"), Bytes::from("2.1")),
             (Bytes::from("c"), Bytes::from("3.1")),
             (Bytes::from("d"), Bytes::from("4.2")),
+            (Bytes::from("e"), Bytes::new()),
         ],
     );
 
@@ -158,6 +160,7 @@ fn test_task2_merge_1() {
             (Bytes::from("b"), Bytes::from("2.3")),
             (Bytes::from("c"), Bytes::from("3.3")),
             (Bytes::from("d"), Bytes::from("4.3")),
+            (Bytes::from("e"), Bytes::new()),
         ],
     );
 }