In QlikView in the object properties on the object Layout tab there is a "Show" option.
Choose conditional and write a formula.
Using the formula above the field will only show if something is selected.
If you need the field to stay visible - even if the user clears all selections - you will need to introduce a variable that is set to 0 when the form is opened, and changed to 1 once something is selected. The conditional statement would then only look at that variable