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

write some css below

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

    display: none;

}

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!

Thanks .