Some functions / methods can generate some sort off entity or part with NaN / Null / Null-lenght ... props.
Some explicitly don't and test their actions with isNull().

As these serve a purpose in some processes, trails of them can be part of the results.
For some users Null / Null-lenght are meaningfull, for other users these produce errors down the line.
In a few cases its a desirable feature eg. controlpoint spline.
(Allthough there are no Null props involved with equal successively controlpoints)

Many problem post on the forum are related to Null - Zero and so on.

In some cases it can't be undone by re-editing the drawing.
FS#1925

If it can't be undone by Undo/Redo it is a bug.
FS#1757

In some cases there is a difference noticed after file save-close-open.