A value of null is not understood

In json "null" is a valid value for an item in a list or object. `json.loads("null")` returns `None`.
Because `next_value` returns `json.loads(value_string)` or `None` (or a Transient subclass), and that value is compared to `None`, a list like [1,2,null,3,4,5] is wrongly interpreted.
Here is a comparison with `json_stream` too.

```py
import adafruit_json_stream
import json_stream
import io

def test_null_item(json_stream):
print("-"*10)
source = io.BytesIO(b'[1,2,null,3,4]')
stream = json_stream.load(source)
for item in stream:
print("-", item)

test_null_item(adafruit_json_stream)
test_null_item(json_stream)
```
```
----------
- 1
- 2
----------
- 1
- 2
- None
- 3
- 4
```