Don't emit hello event if no events were received from stdin

When running `nak req ... relay.one | nak event relay.two`,
if the first req doesn't return any events, the second
nak should not publish a "hello from nostr army knife" note
to the second relay as it is clearly not the intention.

`nak event relay.two` behavior is unchanged, it will publish the hello.

helpers.go

@@ -75,7 +75,7 @@ func getJsonsOrBlank() iter.Seq[string] {
 			return true
 		})
 
-		if !hasStdin {
+		if !hasStdin && !isPiped() {
 			yield("{}")
 		}