The factory method for SameNodeJoinCondition on QueryObjectModelFactory says that selector2Path is mandatory (non-null) while the get method on
SameNodeJoinCondition for selector2Path says it is optional.
I suggest we make it mandatory (non-null). This simplifies JavaDoc for
SameNodeJoinCondition and makes it easier to understand. The null case can easily
be mimicked by a client with a relative path value of "."