Update empty Map to_repr to match syntax

Empty Map will output `[:]` instead of `[]` now

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/map.rs

@@ -135,7 +135,7 @@ impl Map {
         let this_borrowed = this.borrow();
         let map_obj = this_borrowed.as_any().downcast_ref::<Map>().unwrap();
         if map_obj.table().len() == 0 {
-            return Str::create("[]");
+            return Str::create("[:]");
         }
 
         let mut repr = "[".to_string();