Fix pen_width bug in cairo_draw

When user passes pen_width value only for either vertex or edge, an
error is thrown since inside min_lw, lw may be None. This bug fixes
that by assigning value lw=0.1 whenever it is 0.1
3 jobs for master in 0 seconds (queued for 72 minutes and 46 seconds)
latest
Status Job ID Name Coverage
  Test
failed #5001
amd64
job_clang_amd64

failed #5002
amd64
job_clang_cxx_amd64

failed #5000
amd64
job_gcc_amd64

 
Name Stage Failure
failed
job_gcc_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
job_clang_cxx_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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failed
job_clang_amd64 Test There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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