Fix buildDHCPv6Modifiers: DUID added before nil opts guard

[Copilot]

buildDHCPv6Modifiers was unconditionally calling getDUID() before the opts == nil early return, causing TestBuildDHCPv6Modifiers/nil_opts to fail — 1 modifier returned instead of 0.

Change

Move the opts == nil guard to the top of the function. The DUID client-ID modifier is only meaningful alongside actual DHCPv6 options; nil opts means no request will be made.

// Before
func (m *Manager) buildDHCPv6Modifiers(ifaceName string, opts *DHCPv6Options) []dhcpv6.Modifier {
    var mods []dhcpv6.Modifier
    if duid, err := m.getDUID(ifaceName); err == nil { // runs even when opts == nil
        mods = append(mods, dhcpv6.WithClientID(duid))
    }
    if opts == nil {
        return mods // returns [DUID modifier] instead of nil
    }
    ...
}

// After
func (m *Manager) buildDHCPv6Modifiers(ifaceName string, opts *DHCPv6Options) []dhcpv6.Modifier {
    if opts == nil {
        return nil
    }
    var mods []dhcpv6.Modifier
    if duid, err := m.getDUID(ifaceName); err == nil {
        mods = append(mods, dhcpv6.WithClientID(duid))
    }
    ...
}

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