Deal with buggy implicit use of Undefined truthiness

[aaronayres35]

See discussion:
#715 (comment)

one relevant code block is:

enable/enable/tools/drag_tool.py

Lines 155 to 157 in 8d8a4de

	button_down = getattr(event, self.drag_button + "_down")
	if state == "nondrag":
	if (button_down

it is possible the truth value of Undefined is being used elsewhere in the code base, and in general this is something we should aim to avoid.