Skip serializing null app_id in application password params (#1048)

WordPress 6.9-RC3 rejects `app_id: null` in the application-passwords
endpoint, returning "Invalid parameter(s): app_id". This is a regression
from previous versions where null was accepted.

Changes:
- Add `#[serde(skip_serializing_if = "Option::is_none")]` to `app_id` in
  `ApplicationPasswordCreateParams` and `ApplicationPasswordUpdateParams`

Author: oguzkocer

wp_api/src/application_passwords.rs

@@ -53,6 +53,7 @@ pub struct IpAddress {
 pub struct ApplicationPasswordCreateParams {
     /// A UUID provided by the application to uniquely identify it.
     /// It is recommended to use an UUID v5 with the URL or DNS namespace.
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub app_id: Option<String>,
     /// The name of the application password.
     pub name: String,
@@ -74,6 +75,7 @@ pub struct ApplicationPasswordDeleteAllResponse {
 pub struct ApplicationPasswordUpdateParams {
     /// A UUID provided by the application to uniquely identify it.
     /// It is recommended to use an UUID v5 with the URL or DNS namespace.
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub app_id: Option<String>,
     /// The name of the application password.
     pub name: String,