plainbox crashes when job has unknown estimated duration

[Zygmunt Krynicki]

This seems to be caused by the recent patches that work with estimates.

(venv.fx)zyga@fx:~/checkbox$ plainbox run -i 2013.com.canonical.certification::graphics/generator_screenshot_fullscreen_video -i '2013.com.canonical.certification::graphics/[0-9]_screenshot_fullscreen_video.+'

Traceback (most recent call last):
  File "/home/zyga/checkbox/venv.fx/bin/plainbox", line 9, in <module>
    load_entry_point('plainbox==0.21.dev0', 'console_scripts', 'plainbox')()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/box.py", line 122, in main
    raise SystemExit(PlainBoxTool().main(argv))
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 324, in main
    return self.dispatch_and_catch_exceptions(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 560, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 556, in dispatch_command
    return ns.command.invoked(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/cmd_run.py", line 45, in invoked
    self.provider_loader, self.config_loader, ns, ns.color).run()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/inv_run.py", line 358, in run
    return self.do_normal_sequence()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/inv_run.py", line 400, in do_normal_sequence
    self.run_all_selected_jobs()
  File "/home/zyga/checkbox/plainbox/plainbox/impl/commands/inv_run.py", line 724, in run_all_selected_jobs
    estimated_time += job.estimated_duration
TypeError: unsupported operand type(s) for +=: 'float' and 'NoneType'