fix: skip history if empty input

src/simple_repl.gleam

@@ -55,11 +55,18 @@ fn append_history(cmd: String) -> Nil {
 fn repl_loop(cmds: dict.Dict(String, fn() -> Nil)) -> a {
   let input = get_line("repl> ") |> string.trim
   let cmd = string.lowercase(input)
+
+  case cmd {
+    "" -> Nil
+    _ -> {
       input |> append_history
       case dict.get(cmds, cmd) {
         Ok(func) -> func()
         Error(_) -> io.println(input <> ": unknown command")
       }
+    }
+  }
+
   repl_loop(cmds)
 }