id_emp.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence charSequence, int start, int i1, int i2) {

        }
        @Override
        public void onTextChanged(CharSequence charSequence, int i, int before, int count) {
            if (count>0){ //count es cantidad de caracteres que tiene
                buttonOk.setEnabled(true);
            }else{
                buttonOk.setEnabled(false);
            }
        }
        @Override
        public void afterTextChanged(Editable editable) {

        }
    });