Updated:
ModeShape does not provide an easy way to find nodes that have no children. There are several ways to try, including an anti-join that does not work (this was the original topic of this issue; see below). The best way to solve this problem is to provide a new dynamic operand that can be used to constrain the number of children. For example,

    ... WHERE CHILDCOUNT('selector') = 0 ...

Original:
ModeShape does not appear to properly handle returning only the rows in the LEFT OUTER JOIN for which there is a non-null value on the left and a null on the right. (The LEFT INNER JOIN will return only the rows for which there is a non-null value on both the left and the right.)