Fix a crash when completing empty line

lib/reline/line_editor.rb

@@ -1295,8 +1295,10 @@ def editing_mode
       end
     end
     completed = @completion_journey_data.list[@completion_journey_data.pointer]
-    @line = (@completion_journey_data.preposing + completed + @completion_journey_data.postposing).split("\n")[@line_index]
+    new_line = (@completion_journey_data.preposing + completed + @completion_journey_data.postposing).split("\n")[@line_index]
+    @line = new_line.nil? ? String.new(encoding: @encoding) : new_line
     line_to_pointer = (@completion_journey_data.preposing + completed).split("\n").last
+    line_to_pointer = String.new(encoding: @encoding) if line_to_pointer.nil?
     @cursor_max = calculate_width(@line)
     @cursor = calculate_width(line_to_pointer)
     @byte_pointer = line_to_pointer.bytesize

test/reline/yamatanooroti/test_rendering.rb

@@ -823,6 +823,19 @@ def test_completion_journey_2nd_line
       EOC
     end
 
+    def test_completion_journey_with_empty_line
+      write_inputrc <<~LINES
+        set editing-mode vi
+      LINES
+      start_terminal(10, 50, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --complete}, startup_message: 'Multiline REPL.')
+      write("\C-n\C-p")
+      close
+      assert_screen(<<~'EOC')
+        Multiline REPL.
+        prompt>
+      EOC
+    end
+
     def write_inputrc(content)
       File.open(@inputrc_file, 'w') do |f|
         f.write content