Fix drawing of graphs with no edges

This fixes issue #269
8 jobs for master
Status Job ID Name Coverage
  Test
passed #933
amd64
job_clang_amd64

02:58:58

passed #937
amd64
job_clang_amd64_nosh

02:59:22

passed #935
amd64
job_clang_py2_amd64

02:55:22

failed #930
i386
job_gcc_386

00:21:35

passed #932
amd64
job_gcc_amd64

02:12:52

passed #936
amd64
job_gcc_amd64_nosh

02:12:44

failed #931
i386
job_gcc_py2_386

02:44:53

passed #934
amd64
job_gcc_py2_amd64

02:14:41

 
Name Stage Failure
failed
job_gcc_386 Test There is an unknown failure, please try again
Running Sphinx v1.3.5
making output directory...
Unable to init server: Could not connect: Connection refused
Unable to init server: Could not connect: Connection refused

Extension error:
Could not import extension numpydoc (exception: No module named 'numpydoc')

ERROR: Build failed with: exit status 1
failed
job_gcc_py2_386 Test There is an unknown failure, please try again
Doctest summary
===============
860 tests
1 failure in tests
0 failures in setup code
0 failures in cleanup code
build finished with problems, 137 warnings.

ERROR: Build failed with: exit status 1