getDropdownValue specific list options

Hello, I am trying to calculate a subject field to populate the text of dropdown list task_type_list, with the exception of a balnk option- which should allow the subject to be editable.  I've tried a few different formulas and keep coming up incorrect.  When I use the base formula getDropdownValue("task_type_list",$task_type_c) the field will populate with the drop down option selected.  I need to exclude the blank option so that it can be manually typed in the subject line.