chore: initialize channels in tests first when needed, as it was removed from asyncApiDocument

Author: DomKal11

test/LEGO.AsyncAPI.Tests/AsyncApiDocumentBuilder.cs

@@ -2,9 +2,10 @@
 
 namespace LEGO.AsyncAPI.Tests
 {
+    using System;
+    using System.Collections.Generic;
     using LEGO.AsyncAPI.Models;
     using LEGO.AsyncAPI.Models.Interfaces;
-    using System;
 
     internal class AsyncApiDocumentBuilder
     {
@@ -42,6 +43,10 @@ public AsyncApiDocumentBuilder WithDefaultContentType(string contentType = "appl
 
         public AsyncApiDocumentBuilder WithChannel(string key, AsyncApiChannel channel)
         {
+            if (this.document.Channels == null)
+            {
+                this.document.Channels = new Dictionary<string, AsyncApiChannel>();
+            }
             this.document.Channels.Add(key, channel);
             return this;
         }

test/LEGO.AsyncAPI.Tests/AsyncApiDocumentV2Tests.cs

@@ -1202,6 +1202,10 @@ public void Serialize_WithBindingReferences_SerializesDeserializes()
                     },
                 },
             };
+            if (doc.Channels == null)
+            {
+                doc.Channels = new Dictionary<string, AsyncApiChannel>();
+            }
             doc.Channels.Add(
                 "testChannel",
                 new AsyncApiChannel
@@ -1260,6 +1264,10 @@ public void Serializev2_WithBindings_Serializes()
                 Protocol = "pulsar+ssl",
                 Url = "example.com",
             });
+            if (doc.Channels == null)
+            {
+                doc.Channels = new Dictionary<string, AsyncApiChannel>();
+            }
             doc.Channels.Add(
                 "testChannel",
                 new AsyncApiChannel