Added tests to drain an empty vec

Discovered this kind of issue in an unrelated library.
The author copied the tests from here and AFAIK, there are no tests for this particular case.

Signed-off-by: Hanif Bin Ariffin <hanif.ariffin.4326@gmail.com>

library/alloc/tests/vec.rs

@@ -609,6 +609,17 @@ fn test_move_items_zero_sized() {
     assert_eq!(vec2, [(), (), ()]);
 }
 
+#[test]
+fn test_drain_empty_vec() {
+    let mut vec: Vec<i32> = vec![];
+    let mut vec2: Vec<i32> = vec![];
+    for i in vec.drain(..) {
+        vec2.push(i);
+    }
+    assert!(vec.is_empty());
+    assert!(vec2.is_empty());
+}
+
 #[test]
 fn test_drain_items() {
     let mut vec = vec![1, 2, 3];