Update config retrieval method and add null check

Changed the retrieval of `pingTargets` from `GetValue<string[]>("PingTargets")` to `GetSection("PingTargets").Get<string[]>()`. Added a null check to return the `services` object immediately if `pingTargets` is null.

src/Moonglade.IndexNow.Client/ServiceCollectionExtension.cs

@@ -9,7 +9,8 @@ public static class ServiceCollectionExtension
 {
     public static IServiceCollection AddIndexNowClient(this IServiceCollection services, IConfigurationSection configurationSection)
     {
-        var pingTargets = configurationSection.GetValue<string[]>("PingTargets");
+        var pingTargets = configurationSection.GetSection("PingTargets").Get<string[]>();
+        if (null == pingTargets) return services;
 
         foreach (var pingTarget in pingTargets)
         {