fix: Use null-conditional operator to access FinishedAt (#1499)

Author: HofmeisterAn

src/Testcontainers/Containers/DockerContainer.cs

@@ -559,7 +559,7 @@ await _configuration.StartupCallback(this, ct)
 
       Logger.CompleteReadinessCheck(_container.ID);
 
-      StartedTime = DateTime.TryParse(_container.State.StartedAt, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal, out var startedTime) ? startedTime : DateTime.UtcNow;
+      StartedTime = DateTime.TryParse(_container.State!.StartedAt, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal, out var startedTime) ? startedTime : DateTime.UtcNow;
       Started?.Invoke(this, EventArgs.Empty);
     }
 
@@ -595,7 +595,7 @@ await _client.StopAsync(_container.ID, ct)
         _container = new ContainerInspectResponse();
       }
 
-      StoppedTime = DateTime.TryParse(_container.State.FinishedAt, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal, out var stoppedTime) ? stoppedTime : DateTime.UtcNow;
+      StoppedTime = DateTime.TryParse(_container.State?.FinishedAt, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal, out var stoppedTime) ? stoppedTime : DateTime.UtcNow;
       Stopped?.Invoke(this, EventArgs.Empty);
     }