[waterfallplot] Crashes if font has no BlueValues

[miguelsousa]

Traceback (most recent call last):
  File "bin/waterfallplot", line 11, in <module>
    load_entry_point('afdko', 'console_scripts', 'waterfallplot')()
  File "afdko/Tools/SharedData/FDKScripts/ProofPDF.py", line 956, in waterfallplot
    main()
  File "afdko/Tools/SharedData/FDKScripts/ProofPDF.py", line 983, in main
    proofMakePDF(params.rt_fileList, params, txPath)
  File "afdko/Tools/SharedData/FDKScripts/ProofPDF.py", line 889, in proofMakePDF
    pdfFont = otfPDF.txPDFFont(ttFont, params)
  File "afdko/Tools/SharedData/FDKScripts/otfPDF.py", line 84, in __init__
    self.GetBlueZones()
  File "afdko/Tools/SharedData/FDKScripts/fontPDF.py", line 483, in GetBlueZones
    self.blueZones = self.clientGetBlueZones()
  File "afdko/Tools/SharedData/FDKScripts/otfPDF.py", line 176, in clientGetBlueZones
    rawBlueList = fontDict.Private.BlueValues
  File "site-packages/fontTools/cffLib/__init__.py", line 2245, in __getattr__
    raise AttributeError(name)
AttributeError: BlueValues