Remove Graph::isEmpty()

[clue]

Its definition is ambiguous: the current implementation considers a Graph with no vertices as empty, whereas the common (ymmv) interpretation is to check for the number of edges instead.

Once #48 is merged, it should be trivially easy to replace all existing calls to explicitly check for either $graph->getVertices()->isEmpty() or $graph->getEdges()->isEmpty() instead.

[clue]

Fixed via #63.