NumericWidget : Fix up/down key handling when the widget is empty
NumericWidget : Fix up/down key handling when the widget is empty

Fixes
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- Fixed bug that caused an exception to be raised if the up/down keys
  were pressed in a NumericWidget when the widget's value was empty.

hradec/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,hradec/gaffer,ImageEngine/gaffer,boberfly/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,boberfly/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,boberfly/gaffer,hradec/gaffer,hradec/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,hradec/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer