How to select input value from onchange using jQuery?

I have a select and input label. I am updating the value of a text input based on what is selected in the select tag.

select label

<select class="form-control form-field-username form-field-users">
<option class="form-field-first-option" value="0" selected="selected">Select User...</option>
<option value="1">Peter Fel</option> 
<option value="2">Mark Getty</option>
</select>

input tag

<input name="user_id" id="user_id" class="form-control form-field-user-id" type="number">

The code selected by onchange:

$('.form-field-username').change( function() {
    $(this).find(":selected").each(function () {
        var userId = $(this).val();

        $('.form-field-user-id').val(userId);

    });

});

I want to set the value of the text input with user id "user_id" to null or undefined if onchange selects the first value or user_id == 0.

Do you know how to modify the code? Any help is appreciated. thanks