[ruby/reline] Avoid tripping over nil prompt

https://github.com/ruby/reline/commit/d4d9d3e3d4

lib/reline/line_editor.rb

@@ -124,6 +124,7 @@ class Reline::LineEditor
         @prompt_cache_time = Time.now.to_f
       end
       prompt_list.map!{ prompt } if @vi_arg or @searching_prompt
+      prompt_list = [prompt] if prompt_list.empty?
       mode_string = check_mode_string
       prompt_list = prompt_list.map{ |pr| mode_string + pr } if mode_string
       prompt = prompt_list[@line_index]