I have a predicate map that I am adding a orderby value to and sorting ascending on that field using orderby.sort.
I need to sort ascending on that field, while also adding a "nulls last" sort so that any sortable values will be sorted ascending at the beginning of the list, while all items with a null value in that sortable field will then show up in the list after that. 
How can I accomplish this?
Example:

currentParamMap.put("type", "cq:Page");
currentParamMap.put("orderby", "@jcr:content/publishDate");
currentParamMap.put("orderby.sort", "asc");

I have tried to add a second orderby via 1_orderby and 2_orderby and included the two sort values as 1_orderby.sort "asc" and 2_orderby.sort "nulls last" This does not work for me. I can sort by one or the other, but not both. Can this be done?