Handle the case of an unreachable vertex

If d is unreachable from s, return a table with just one line:
    EDGE_GEOM = null
    EDGE_ID = -1
    PATH_ID = -1
    PATH_EDGE_ID = -1
    SOURCE = s
    DESTINATION = d
    WEIGHT = Double.POSITIVE_INFINITY

Also test when a target vertex is not found.

TODO: Test when a source vertex is not found.
Add test for dest vertex