Unit test debug print of witness with empty instruction

We recently fixed a bug that causes a panic if a `Witness` contains an
empty instruction. Add a unit test to verify it.

bitcoin/src/blockdata/witness.rs

@@ -535,6 +535,16 @@ mod test {
         v
     }
 
+    #[test]
+    fn witness_debug_can_display_empty_instruction() {
+        let witness = Witness {
+            witness_elements: 1,
+            content: append_u32_vec(vec![], &[0]),
+            indices_start: 2,
+        };
+        println!("{:?}", witness);
+    }
+
     #[test]
     fn test_push() {
         let mut witness = Witness::default();