Both JMC and the JFR parser reports that some events from non-java threads have a 'null' eventThread.

I found that the following code fails to checkpoint the non-java threads:
void JfrThreadConstantSet::serialize(JfrCheckpointWriter& writer) {
  JfrCheckpointThreadClosure tc(writer);
...
  JfrNonJavaThreadIterator nonjavathreads;
  while (nonjavathreads.has_next()) {
    tc.do_thread(nonjavathreads.next());
  }
}

because the following code only returns the last thread:
---
static NonJavaThread* next_non_java_thread(NonJavaThread::Iterator& iter) {
  NonJavaThread* next = NULL;
  while (!iter.end()) {
    next = iter.current();
    iter.step();
    assert(next != NULL, "invariant");
    if (!thread_inclusion_predicate(next)) {
      continue;
    }
  }
  return next;
}
---

I've tested with the following code, and then we get the correct thread names:
---
static NonJavaThread* next_non_java_thread(NonJavaThread::Iterator& iter) {
  while (!iter.end()) {
    NonJavaThread* next = iter.current();
    iter.step();
    assert(next != NULL, "invariant");
    if (thread_inclusion_predicate(next)) {
      return next;
    }
  }
  return NULL;
}
---