"straighten up" extract_session

Ran into while reviewing #669 and it is OOS for that PR so filing separately

We have
```
def extract_session(metadata: dict) -> list:
```
but that function
- can return `None`, so I guess it should become
```
def extract_session(metadata: dict) -> Optional[list]:
```
- we return a list but have `extract_session` not `sessions`. Should we return a singular `models.Session` and listify it outside may be so that name is more consistent with return value?

WDYT @satra @jwodder ?