In my case I really need to have that Tk::Entry to be never empty.
The reason is following.
I tried to use same text variable for this Tk::Entry and for other widget Tk::Scale to allow for user to either enter number by hand or to drag scale widget and enter value "visually".

The point is - that works fine! And it seemed to me to be quite elegant solution.
And validate works okay, but once Entry became empty (even for just a moment, when that extra validation succeedes) then strange things happen: Scale widget explains that it could not have empty value, then someone prints some Tcl_panic message, garbage could appear after some manipulation and after some other GUI manipulations it could even core dump!

No coredumps and garbage inside Tk::Entry was noticed in my GUI app otherwise.

So, I'm still trying to find a way to solve this!

Best wishes,
I.R.Baboon.