Protect against crash in the `relation_possible` predicate with ambiguous relations. It was still possible to get a KeyError when using relations that are ambiguous at both ends, like (Executable, version_of, Program) & (Version, version_of, Project). Closes #3010148.

rql('SET X name "BoundaryConstraint" '
    'WHERE X is CWConstraintType, X name "BoundConstraint"')