Tclkit does this
% tclkit
% set null \x0000
�
%
instead of this
% tclsh
% set null \x0000
%

With Tclkit 8.4.6 on Linux, I get:
$ tclkit
% set null \x0000
??
%
% string length $null
1
% encoding system
iso8859-1
%
I'm not sure how to interpret this.