Unconnected graphs are not drawn

[teake]

Hi,

Thanks for the nice library! I did find one issue though: graphs with only unconnected vertices are not drawn. The simplest case (below) yields ''. Graphs with both connected and unconnected vertices do get drawn ok though.

import networkx as nx
from asciinet import graph_to_ascii

G = nx.Graph()
G.add_node(1)
assert graph_to_ascii(G) != ''