Remove TODOs in Ruby seralizer

All issues mentioned in removed TODOs are now tracked on our Trello
board:

  https://trello.com/b/ocGaf1lE
Formatting: Use trailing form for |if| statements when possible
Formatting: Use trailing form for |unless| statements when possible
Formatting: Never split an empty array to multiple lines
Formatting: Never split an empty hash to multiple lines
Formatting: Never split an empty expression list to multiple lines