Box Plot widget: if a feature have no data, widget produce error

[brunap]

Orange version

3.3.7

Expected bahavior

If a selected feature has no data, box plot widget should return warning.

Actual behavior

If a feature with no data is selected in the Variable box, Orange returns error.

Additional info (error data)

ValueError Traceback (most recent call last):
File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/Orange/widgets/gui.py", line 2440, in call
self.func(**kwds)
File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/Orange/widgets/visualize/owboxplot.py", line 247, in attr_changed
self.layout_changed()
File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/Orange/widgets/visualize/owboxplot.py", line 318, in layout_changed
self.draw_axis()
File "/Applications/Orange3.app/Contents/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/Orange/widgets/visualize/owboxplot.py", line 512, in draw_axis
bottom = min(stat.a_min for stat in self.stats)
ValueError: min() arg is an empty sequence