RFR: 8278826: Print error if Shenandoah flags are empty (instead of crashing)

[Hamlin Li]

On Tue, 14 Dec 2021 23:11:46 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

> ShouldNotReachHere() is replaced by vm_exit_during_initialization() in few places where ShenandoahGCMode and ShenandoahGCHeuristics are examined.
> 
> Generic testing: gc/shenandoah jtreg tests.

Looks good.
Seems by changing `if (ShenandoahGCHeuristics != NULL)` to `if (ShenandoahGCHeuristics == NULL)`, the trailing `return NULL;` could be removed.

-------------

Marked as reviewed by mli (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6842