driver: don't expect nodes to exist when checking paths between them

In mgQueryZero, previously node lookups were expected to never fail, i.e. it was expected that when calculating the path between two nodes in a zero level import graph, both nodes would always exist. This is not the case, e.g. in some situations involving exact names (see the test-case). The fix is to first check whether the node is present in the graph at all, instead of panicking, just to report that there is no path.

@sheaf recommended doing two lookups to not change the semantics of mgQueryZero but I think that this is an actual improvement of the logic and in accordance of what the documentation describes this function is supposed to do - the code path that was causing the panic here is also the only code path that calls this function at all, so it should be extra okay.

Closes #26568 (closed)