Fix repl feedback

repl/repl.go

@@ -164,7 +164,7 @@ func (r *REPL) StartEvalLoop(in io.Reader, out io.Writer, env *object.Environmen
 		}
 
 		obj := evaluator.Eval(program, env)
-		if obj != nil {
+		if _, ok := obj.(*object.Null); !ok {
 			io.WriteString(out, obj.Inspect())
 			io.WriteString(out, "\n")
 		}
@@ -216,9 +216,11 @@ func (r *REPL) StartExecLoop(in io.Reader, out io.Writer, state *VMState) {
 			return
 		}
 
-		stackTop := machine.LastPoppedStackElem()
-		io.WriteString(out, stackTop.Inspect())
-		io.WriteString(out, "\n")
+		obj := machine.LastPoppedStackElem()
+		if _, ok := obj.(*object.Null); !ok {
+			io.WriteString(out, obj.Inspect())
+			io.WriteString(out, "\n")
+		}
 	}
 }