allow rule to raise NoResult to indicate that for the given inputs, a particular rule can not return an output

This is useful in the case of a SQLQuery where the constraint is moved into the query and out of graphcore. In this case the graphcore rule needs to be able to tell graphcore that the inputs have no valid output and so filter the results. There are other cases where this may be desirable. For example, in the query:

```
{
'user.id': 1,
'user.parent.name?': None,
}
```

it is possible that the user has no parent entered and so just by asking for the parent you are constraining the set of users.