Null DiGraph

This is a tiny point, but just for record:

• Should graph_tools.DiGraph allow a null graph (a graph with no node)? (It is allowed in the current implementation.)
• If so, is a null digraph strongly connected? (It is not in the current implementation.)

>>> adj_matrix = np.array([]).reshape(0, 0)
>>> adj_matrix
array([], shape=(0, 0), dtype=float64)
>>> null_digraph = qe.DiGraph(adj_matrix)
>>> null_digraph
Directed Graph:
  - n(number of nodes): 0
>>> null_digraph.is_strongly_connected
False
>>> null_digraph.num_strongly_connected_components
0
>>> null_digraph.strongly_connected_components
[]
>>> null_digraph.num_sink_strongly_connected_components
0
>>> null_digraph.sink_strongly_connected_components
[]

In passing, in Networkx whether or not a null digraph is strongly connected is undefined.

>>> import networkx as nx
>>> G = nx.DiGraph()
>>> G.number_of_nodes()
0
>>> nx.is_strongly_connected(G)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 2, in is_strongly_connected
  File "/usr/local/lib/python2.7/site-packages/networkx/utils/decorators.py", line 68, in _not_implemented_for
    return f(*args,**kwargs)
  File "/usr/local/lib/python2.7/site-packages/networkx/algorithms/components/strongly_connected.py", line 287, in is_strongly_connected
    """Connectivity is undefined for the null graph.""")
networkx.exception.NetworkXPointlessConcept: Connectivity is undefined for the null graph.
>>> nx.number_strongly_connected_components(G)
0
>>> list(nx.strongly_connected_components(G))
[]

Start with graph_tools.DiGraph and the is_strongly_connected, num_strongly_connected_components, strongly_connected_components, and sink-component entry points described in the issue. First establish the intended convention for null digraphs, then verify that the implementation consistently follows that decision. Done means the null-graph policy is decided and its observable behavior is consistent.