Make 0.radical() return 0 #41222
Description
As in the title. Currently it raises ArithmeticError, which leads to QQ.nilradical(), ZZ.nilradical() etc. complains as well.
(should we?)
The rationale is that the radical of the zero ideal is zero (in rings without nilpotent elements), and R.ideal(x).radical() == R.ideal(x.radical()) holds for all UFD R.