[foreign] RFR 8219119: Need API to create a null Callback

[Sundararajan Athijegannathan]

In response to -> 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-February/004374.html

Updated webrev: https://cr.openjdk.java.net/~sundar/8219119/webrev.01/

Thanks,
-Sundar