An empty or blank box appears inside List collector in Service Portal

If the list collector field shows an empty box, this means any empty character is passed in the field via client script or UI policies via setValue().

As shown in the following screenshot, when an empty box shows in the list collector field it means your client script is using a space character to set the value.

Identify the client scripts used to set the value for the list collector variable

This example shows the extra space in the g_form.setValue() 

g_form.setValue('affected_varibale_name',' ');

To fix the issue, remove the empty space used in the setValue() 

g_form.setValue('affected_varibale_name','');