Use Task.CompletedTask (#789)

Author: WeihanLi

src/KubernetesClient/Autorest/BasicAuthenticationCredentials.cs

@@ -49,7 +49,7 @@ public override Task ProcessHttpRequestAsync(
                     "{0}:{1}",
                     UserName,
                     Password).ToCharArray())));
-            return Task.FromResult<object>(null);
+            return Task.CompletedTask;
         }
     }
 }

src/KubernetesClient/Autorest/ServiceClientCredentials.cs

@@ -23,7 +23,7 @@ public abstract class ServiceClientCredentials
         public virtual Task ProcessHttpRequestAsync(HttpRequestMessage request, CancellationToken cancellationToken)
         {
             // Return an empty task by default
-            return Task.FromResult<object>(null);
+            return Task.CompletedTask;
         }
     }
 }