Version 8.23 of SSMA SQL Server Migration Assistant for Access is available.

https://www.microsoft.com/en-us/download/details.aspx?id=54255

Beginning with 8.13 SSMA is able to automatically create unique indizes containing several NULL values by creating a so called filtered index in SQL Server.

This of course does NOT apply to PKs (=Primary Keys), just to "normal" unique indices with serveral NULL values. 

This was declared from the MS SSMA-feedback team as a reply to my explicit question. So the documentation in SSMA pages is outdated in that special point.

mfg Klaus