structs: fix test for empty DNS configuration (#20233)

The `DNSConfig.IsZero` method incorrectly returns true if any of the fields are
empty, rather than if all of them are empty.

The only code path that consumes this method is on the client, where it's used
as part of equality checks on the allocation network status to set the priority
of allocation updates to the server. Hypothetically, if the network hook
modified only the DNS configuration and no task states were emitted, it would be
possible to miss an allocation update. In practice this appears to be
impossible, but we should fix the bug so that there aren't errors in future
consumers.

nomad/structs/structs.go

@@ -2851,7 +2851,7 @@ func (d *DNSConfig) IsZero() bool {
 	if d == nil {
 		return true
 	}
-	return len(d.Options) == 0 || len(d.Searches) == 0 || len(d.Servers) == 0
+	return len(d.Options) == 0 && len(d.Searches) == 0 && len(d.Servers) == 0
 }
 
 // NetworkResource is used to represent available network

nomad/structs/structs_test.go

@@ -7482,6 +7482,7 @@ func TestDNSConfig_Equal(t *testing.T) {
 
 	must.Equal[*DNSConfig](t, nil, nil)
 	must.NotEqual[*DNSConfig](t, nil, new(DNSConfig))
+	must.NotEqual[*DNSConfig](t, nil, &DNSConfig{Servers: []string{"8.8.8.8"}})
 
 	must.StructEqual(t, &DNSConfig{
 		Servers:  []string{"8.8.8.8", "8.8.4.4"},