Reservations prints one blank reservation if no reservations specified

If no reservations are specified, then one blank reservation like this:

        "reservations": [
            {
                "hw-address": "",
                "hostname": "",
                "client-classes": [
                    ""
                ]
            }
        ],

is added. This causes the error:

2024-01-22 20:12:58.190 ERROR [kea-dhcp4.dhcp4/1304.281473680973840] DHCP4_PARSER_FAIL failed to create or run parser for configuration element reservations: empty host identifier used (test.json:78:13)

Disable addition of blank reservations section.

No need for a release note as this feature was introduced in this same version.