Promise: do not track handled rejections with no list

Promise: do not track handled rejections with no list

Previously, when a handled promise was reported and the rejected promise
list was NULL, the trackers fell through and added it as unhandled.  In
the interactive shell, handling a promise after its rejection had been
reported caused another, false unhandled rejection.  The QuickJS nginx
module could do the same while formatting a rejection reason.

The fix is to return for every handled notification, removing the promise
from the list only when the list exists.  Apply this to both engines in the
shell and nginx module.