Raise NoneType is not iterable on empty file (e.g. __init__.py)

[davidfischer-ch]

Tested with coverage 4.0b2.
Works well with coverage 3.7.1.

Traceback (most recent call last):
  File "setup.py", line 153, in <module>
    url='https://github.com/davidfischer-ch/pytoolbox'
  File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
    dist.run_commands()
  File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "/usr/lib/python2.7/dist-packages/setuptools/command/test.py", line 135, in run
    self.with_project_on_sys_path(self.run_tests)
  File "/usr/lib/python2.7/dist-packages/setuptools/command/test.py", line 116, in with_project_on_sys_path
    func()
  File "/usr/lib/python2.7/dist-packages/setuptools/command/test.py", line 160, in run_tests
    testLoader = cks
  File "/usr/lib/python2.7/unittest/main.py", line 94, in __init__
    self.parseArgs(argv)
  File "/usr/lib/python2.7/unittest/main.py", line 149, in parseArgs
    self.createTests()
  File "/usr/lib/python2.7/unittest/main.py", line 158, in createTests
    self.module)
  File "/usr/lib/python2.7/unittest/loader.py", line 130, in loadTestsFromNames
    suites = [self.loadTestsFromName(name, module) for name in names]
  File "/usr/lib/python2.7/unittest/loader.py", line 115, in loadTestsFromName
    test = obj()
  File "/home/david/github/davidfischer-ch/pytoolbox/tests/pytoolbox_runtests.py", line 67, in main
    ignore='session.py|schema.py')  # Ignore ming FIXME filter by package
  File "/home/david/github/davidfischer-ch/pytoolbox/pytoolbox/unittest.py", line 56, in runtests
    return nose.main(argv=nose_options)
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/core.py", line 121, in __init__
    **extra_args)
  File "/usr/lib/python2.7/unittest/main.py", line 95, in __init__
    self.runTests()
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/core.py", line 207, in runTests
    result = self.testRunner.run(self.test)
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/core.py", line 66, in run
    result.printErrors()
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/result.py", line 110, in printErrors
    self.config.plugins.report(self.stream)
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/plugins/manager.py", line 99, in __call__
    return self.call(*arg, **kw)
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/plugins/manager.py", line 167, in simple
    result = meth(*arg, **kw)
  File "/usr/local/lib/python2.7/dist-packages/nose-1.3.7-py2.7.egg/nose/plugins/cover.py", line 196, in report
    self.coverInstance.html_report(modules, self.coverHtmlDir)
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/control.py", line 1016, in html_report
    return reporter.report(morfs)
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/html.py", line 133, in report
    self.report_files(self.html_file, morfs, self.config.html_dir)
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/report.py", line 83, in report_files
    report_fn(fr, self.coverage._analyze(fr))
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/html.py", line 178, in html_file
    this_hash = self.file_hash(source.encode('utf-8'), fr)
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/html.py", line 169, in file_hash
    self.coverage.data.add_to_hash(fr.filename, m)
  File "/home/david/github/davidfischer-ch/pytoolbox/coverage-4.0b2-py2.7-linux-x86_64.egg/coverage/data.py", line 583, in add_to_hash
    hasher.update(sorted(self.lines(filename)))
TypeError: 'NoneType' object is not iterable