How to write an empty DateFormat?

Question

Kotlin

What to write in place of "What's here?" to pass an empty value?

(...)
override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
                mListener?.loginFragmentDataListener("",s.toString(),"","","",что тут?)
            }
        })

        return view
    }

    interface LoginFragmentDataListener {
        fun loginFragmentDataListener(
            phone: String,
            firstName: String,
            lastName: String,
            patronymName: String,
            eMail: String,
            dateFormat: DateFormat
        )
    }