Revert "Cleanup/remove empty return values, use i_CTRL-O instead"

`<C-R>` with an empty string as return value is actually required to
make chaining work, as with:

    fun! My_CR_map()
      " "<CR>" via delimitMateCR
      return "\<Plug>delimitMateCR\<Plug>CursorCrossCR\<Plug>DiscretionaryEnd"
    endfun
    imap <expr> <CR> My_CR_map()

Otherwise also autoindent will not happen.

This reverts commit 0d53cf3c307808e59ca32e42be4544165b3bd282.