Make spotter optional.

src/main.rs

@@ -61,7 +61,7 @@ struct Cli {
     
     /// Spotter callsign (e.g. W1CDN)
     #[arg(short = 's', long)]
-    spotter: String,
+    spotter: Option<String>,
 }
 
 /// Convert a byte slice into a hex-dump string for debugging purposes.