Fix error that occurs when close_window_on_exit = false (#336)

Fix error that happens when `close_window_on_exit = false`

```log
E5108: Error executing lua $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:46: Vim:E475: Invalid argument: expected dictionary
stack traceback:
  [C]: in function 'error'
  $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:46: in function 'create'
  $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:60: in function 'create_on_new_window'
  $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:178: in function 'repl_for'
  $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:219: in function 'send'
  $HOME/.vim/plugged/iron.nvim/lua/iron/core.lua:245: in function 'send_line'
  [string ":lua"]:1: in main chunk
```

Co-authored-by: Henry John Kupty <hkupty@users.noreply.github.com>

lua/iron/lowlevel.lua

@@ -72,6 +72,8 @@ ll.create_repl_on_current_window = function(ft, repl, bufnr, current_bufnr, opts
       end
       vim.api.nvim_buf_delete(bufnr, {force = true})
     end
+  else
+    opts.on_exit = function() end
   end
 
   local cmd = repl.command