RFR (XS) 8247220: Make OopHandle constructor explicit

[Kim Barrett]

> On Jun 9, 2020, at 10:36 AM, coleen.phillimore at oracle.com wrote:
> 
> 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

Looks good.