fix: nil: Fix missing field 'merge' initializer for the new cfg_clausedef_t

In !11121, a .merge member was added to cfg_clausedef_t.  This caused
a build failure with -Werror,-Wmissing-field-initializers enabled.
Add the missing initializer and set them all to NULL to match the
intent.

Merge branch 'ondrej/fix-compilation-on-macos' into 'main'

See merge request isc-projects/bind9!11302