Fix ExtRepOfPolynomial_String for zero polynomial

[TWiedemann]

Fixes #5986: String(a) for a rational function a with zero numerator throws an error. The problem was that ExtRepOfPolynomial_String normally returns a mutable string, but for a zero polynomial it used to return String(zero) which is immutable. Hence I changed this to ShallowCopy(String(zero)).

With this change, the example from #5986 works:

gap> x1 := Indeterminate(Rationals, 1);; x2 := Indeterminate(Rationals, 2);; x3 := Indeterminate(Rationals, 3);;
gap> a := (x1*x2)/(x1*x3) - x2/x3;
0/x_1
gap> String(a);
"0/x_1"

The issue that a is represented as 0/x_1 and not as 0 persists.

Text for release notes

none

[fingolfin]

Thank you! That's a good change. Could you please apply the same change to hpcgap/lib/ratfun.gi, and also add a test case? E.g. in tst/testinstall/ratfun.tst or in a new file in tst/testbugfix/ >

[TWiedemann]

Thank you! That's a good change. Could you please apply the same change to hpcgap/lib/ratfun.gi, and also add a test case? E.g. in tst/testinstall/ratfun.tst or in a new file in tst/testbugfix/ >

Done.