About the case where `k=0` is input to `lucasu_mod` and `lucasv_mod`

[haru-44]

For lucasu_mod and lucasv_mod, it seems to be acceptable to input k=0 according to the description.

https://github.com/aleaxit/gmpy/blob/125310619ffc3cbf2a88a3b9121ea74b8e295ec3/src/gmpy_mpz_lucas.c#L185-L188

However, it does not seem to be expected that k=0 is actually input.

[skirpichev]

Same for lucasu/v, they just spin forever.

[casevh]

Since it looks like it works for k>0, I'll just check for k==0 and return the correct value.

For reference, see https://en.wikipedia.org/wiki/Lucas_sequence