Fix Enum required/not required declaration validation

Currently, this maps to not null: 

```python
'role'      : [ HostRole, True, None ],
```
And this maps to null: 

```python
'role'      : [ HostRole, None, None ],
```
The system fails to process this: 

```python
'role'      : [ HostRole, False, None ],
```
Make this behave more like OAGs, with True/False driving null/not null SQL statement, and None being an invalid value. 