[rllib] Simplex action space bug

When trying to initialise the Simplex action space I get the below error

`AttributeError: 'NoneType' object has no attribute 'seed'`

To reproduce the problem exactly you can run:

```
!pip3 install ray[rlib]==0.8.6
!pip3 install dm-tree
from ray.rllib.utils.spaces.simplex import Simplex
Simplex(shape=(4,))
```