[9.18] fix: dev: Add and use __attribute__((nonnull)) in dnssec-signzone.c

Clang 20 was spuriously warning about the possibility of passing a NULL file pointer
to `fprintf()`, which uses the 'nonnull' attribute. To silence the warning, the functions
calling `fprintf()` have been marked with the same attribute to assure that NULL can't be
passed to them in the first place.

Close #5487

Backport of MR !10888

Merge branch 'backport-5487-mark-passed-file-pointer-as-nonnull-in-dnssec-signzone-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10914