add more helper methods

Pilz.Net/Api/ApiMessage.cs

@@ -2,4 +2,9 @@
 
 public abstract class ApiMessage
 {
+    private class EmptyApiMessage : ApiMessage { }
+
+    public ApiMessage Empty { get; } = new EmptyApiMessage();
+
+    public bool IsEmpty => this is EmptyApiMessage;
 }

Pilz.Net/Api/ApiResponse.cs

@@ -5,9 +5,11 @@ namespace Pilz.Net.Api;
 public record class ApiResponse(
     HttpStatusCode StatusCode)
 {
+    public bool IsOk => StatusCode != HttpStatusCode.OK;
+
     public void EnsureOk()
     {
-        if (StatusCode != HttpStatusCode.OK)
+        if (!IsOk)
             throw new Exception("Api return is not ok!");
     }
 }

Pilz.Net/Api/ApiResponse{T}.cs

@@ -8,10 +8,13 @@ public record class ApiResponse<T>(
     T? Message)
     where T : ApiMessage
 {
+    [MemberNotNullWhen(true, nameof(Message))]
+    public bool IsOk => StatusCode == HttpStatusCode.OK && Message is not null;
+
     [MemberNotNull(nameof(Message))]
     public void EnsureOk()
     {
-        if (StatusCode != HttpStatusCode.OK || Message is null)
+        if (!IsOk)
             throw new Exception("Api return is not ok!");
     }
 }