Display list iteration should automatically stop if an item is null

[Gabriel Nava Marino]

We should consider modifying DisplayList::Iterator::operator== so display list iteration automatically stops iterating if an item is null.

As was previously recommended, this can be done by making DisplayList::Iterator::operator== return true when called with a null iterator and the end iterator by changing
```
bool operator==(const Iterator& other) const { return &m_displayList == &other.m_displayList && m_cursor == other.m_cursor; }
```

to
```
bool operator==(const Iterator& other) const
{
    if (atEnd() && other.atEnd())
        return true;
    return &m_displayList == &other.m_displayList && m_cursor == other.m_cursor;
}
```

This requires updating the following two API tests:
- TestWebKitAPI.DisplayListTests.InlineItemValidationFailure
- TestWebKitAPI.DisplayListTests.OutOfLineItemDecodingFailure

[Radar WebKit Bug Importer]

<rdar://problem/86786410>