Prevent parsing error for empty client commands

app/controllers/submissions_controller.rb

@@ -184,6 +184,8 @@ class SubmissionsController < ApplicationController
           # Check whether the client send a JSON command and kill container
           # if the command is 'client_kill', send it to docker otherwise.
           begin
+            next if data == "\n"
+
             parsed = JSON.parse(data)
             if parsed.class == Hash && parsed['cmd'] == 'client_kill'
               Rails.logger.debug("Client exited container.")