Unconnected graphs are not drawn

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.

```python
import networkx as nx
from asciinet import graph_to_ascii

G = nx.Graph()
G.add_node(1)
assert graph_to_ascii(G) != ''
```

