In the service portal, if an empty box showing up in the list collector field, this means any empty character is passed in the field via client script/UI policies via setValue().

As shown in the below screenshot, if you observe an empty box in the list collector field of the service portal. It means your client script is using a space character to set the value.

Identify the client script(s) used to set the value for the list collector variable

You'll observe the g_form.setValue() as below

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

 Remove the empty space used it the setValue() and this will fix the issue

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