support optional server fields

opnsense-config-xml/src/data/haproxy.rs

@@ -310,7 +310,7 @@ pub struct Frontend {
     pub bind_options: MaybeString,
     pub mode: String,
     #[yaserde(rename = "defaultBackend")]
-    pub default_backend: String,
+    pub default_backend: Option<String>,
     pub ssl_enabled: i32,
     pub ssl_certificates: MaybeString,
     pub ssl_default_certificate: MaybeString,
@@ -543,8 +543,8 @@ pub struct HAProxyServer {
     pub enabled: u8,
     pub name: String,
     pub description: MaybeString,
-    pub address: String,
-    pub port: u16,
+    pub address: Option<String>,
+    pub port: Option<u16>,
     pub checkport: MaybeString,
     pub mode: String,
     pub multiplexer_protocol: MaybeString,