fix(demo): avoid error message when the line is empty

Author: javier-godoy

src/test/java/com/flowingcode/vaadin/addons/xterm/XtermDemoView.java

@@ -96,7 +96,9 @@ public XtermDemoView() {
               showHistory();
               break;
             default:
-              xterm.writeln("Bad command");
+              if (!ev.getLine().trim().isEmpty()) {
+                xterm.writeln("Bad command");
+              }
               Notification.show(ev.getLine());
           }
         });