Support rules without positive atoms

Currently, rules require at least one positive atom. This means that e.g.

```
HeapAllocation_Type(?heap, ?type) :-
?heap = "<>",
?type = "java.lang.String[]".
```
(see issue #460) are rejected.

The issue names no file or test. Start by locating the rule validation that rejects rules without positive atoms, then trace how the example in the issue is parsed and evaluated. Done means the shown rule is accepted without regressing rules that contain positive atoms.

Written by the indexing model from the issue text.