Deal with buggy implicit use of Undefined truthiness

[aaronayres35]

See discussion: https://github.com/enthought/enable/pull/715#issuecomment-799483780

one relevant code block is: https://github.com/enthought/enable/blob/8d8a4de91c2396c8f25e2ca91ceffcc580b81fa8/enable/tools/drag_tool.py#L155-L157

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.