G1: remove unnecesary null check for G1ParScanThreadStateSet::_states slots

In `G1ParScanThreadStateSet::flush`:

```
  for (uint worker_id = 0; worker_id < _n_workers; ++worker_id) {
    G1ParScanThreadState* pss = _states[worker_id];

    if (pss == NULL) {
      continue;
    }
    ...
```

A similar `if` check exists in `G1ParScanThreadStateSet::record_unused_optional_region`.

However, `_states` array is lazy-initialized by each worker thread, so all array slots contain non-null value.

Replace the if with a non-null assert.