Cannot output null value in ResultPath

We cannot output the following JSON definition even if result_path is None.

```
{
  "Type": "Parallel",
  "ResultPath": null
}
```

If `result_path` is specified explicitly, user should expect to have it in the final statemachine definition.

```
from stepfunctions import steps

p = steps.Parallel('test', result_path=None)
assert 'ResultPath' in p.to_dict()
```

