So basically, you want to use {title_sort} instead of {title}, but ONLY if there is no series?
(Keep in mind that "a/an/the" will not be used in the sort order on (I think) any device. But if it is visual aesthetics you want, then go ahead. 
)
Try
Code:
{#series_sort}{#series_sort_index:| - | - }{:'test(field("#series_sort"),field("title"),field("title_sort"))'}