new: dev: Add dns_name_empty() to avoid counting labels in emptiness checks

Checking whether a DNS name is empty was done with dns_name_countlabels(), which walks every label in the name just to compare the count with zero. The new dns_name_empty() helper checks the name's length directly, and a coccinelle patch converts the existing callers.

Merge branch 'ondrej/add-dns_name_empty' into 'main'

See merge request isc-projects/bind9!12558