Calling graph_draw() with a graph containing no edges will raise "Value Error" exception

To reproduce this issue, please execute the Python script attached to this issue report.
no_edge.py

Command line dump:

ubuntu@ubuntu-VirtualBox:~/Desktop/Jie$ python ./no_edge.py   
Traceback (most recent call last):   
  File "./no_edge.py", line 7, in <module>   
    graph_draw(g, output="./nodes-only.png")   
  File "/usr/lib/python2.7/dist-packages/graph_tool/draw/cairo_draw.py", line 1134, in graph_draw   
    pad, cr)   
  File "/usr/lib/python2.7/dist-packages/graph_tool/draw/cairo_draw.py", line 1308, in fit_to_view   
    font_size, cr)   
  File "/usr/lib/python2.7/dist-packages/graph_tool/draw/cairo_draw.py", line 1277, in get_bb   
    slm = sl.fa.max() * 0.75   
  File "/usr/lib/python2.7/dist-packages/numpy/core/_methods.py", line 17, in _amax   
    out=out, keepdims=keepdims)   
ValueError: zero-size array to reduction operation maximum which has no identity   
ubuntu@ubuntu-VirtualBox:~/Desktop/Jie$   

This issue seems to have similarity to the closed issue #161 (closed)