[5440] libdhcp++ now quietly drops empty host name options from inbound packets

[5440] libdhcp++ now quietly drops empty host name options from inbound packets

src/lib/dhcp/libdhcp++.cc
    LibDHCP::unpackOptions4() - added logic to drop Host Name option
    if when empty

src/lib/dhcp/tests/libdhcp++_unittest.cc
    TEST_F(LibDhcpTest, emptyHostName)  - new unit test

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::processHostnameOption() - removed prior 5440 logic
    to ignore blank hostname options

src/bin/dhcp4/dhcp4_messages.mes
    Removed prior 5440 message