[#4142] Addressed review comments

[#4142] Addressed review comments

Mostly nits. D2ClientMgr::qualifyName() now throws if
partial name is emtpy.

modified:   changelog_unreleased/CVE-2025-11232-catch-empty-sanitized-hostname
modified:   src/bin/dhcp4/dhcp4_messages.cc
modified:   src/bin/dhcp4/dhcp4_messages.mes
modified:   src/bin/dhcp6/dhcp6_messages.cc
modified:   src/bin/dhcp6/dhcp6_messages.mes
modified:   src/lib/dhcpsrv/d2_client_mgr.cc
modified:   src/lib/dhcpsrv/d2_client_mgr.h
modified:   src/lib/dhcpsrv/tests/d2_client_unittest.cc