RFR (XS) 8247220: Make OopHandle constructor explicit

[coleen.phillimore at oracle.com]

Summary: Fix null initializations to explicitly call the OopHandle 
constructor

See bug report for details.  Tested with tier1-3

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8247220.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8247220

Thanks,
Coleen