This is not currently possible without some shenanigans. There are several forum ideas addressing this point (and also why it's hard from a database perspective):

Remove empty option in Reference Selector

Allowing empty value should be optional for reference selector

This post has a possible workaround by using styling - you might need to adjust this a little for the reference selector:

How to remove empty value from drop-down

Need to remove empty field in dropdown

Hi, fagner

1.Reference selector have data source select microflow option

2. retrieve the association data from database using retrieve activity 

3.Take Loop and pass the retrived list

4. Add a condition->what attribute shows on reference selector !=empty 

5. Return the list

I hope this will be help you

Thank you!

write some css below

.mx-compound-control > .form-control > option:nth-child(1) {

    display: none;

}

Thanks .