make say use Option

src/command/say.rs

@@ -1,16 +1,16 @@
 #[derive(Default)]
 pub struct SayCommand {
-    message: String
+    message: Option<String>
 }
 
 impl SayCommand {
     pub fn message(mut self, msg: impl Into<String>) -> Self {
-        self.message = msg.into();
+        self.message = Some(msg.into());
         self
     } 
 
     pub fn build(self) -> super::Command {
-        let message = self.message;
+        let message = self.message.unwrap_or("Hello, world!".to_string());
         let s = format!("say {message}");
         super::Command(s)
     }