chore: Change command parameter type to Optional[Sequence[str]] in CustomContainerTrainingJob

COPYBARA_INTEGRATE_REVIEW=#5162 from haru-256:chore/fix-type-error 3628505
PiperOrigin-RevId: 747533313

Author: haru-256

google/cloud/aiplatform/training_jobs.py

@@ -4304,7 +4304,7 @@ def __init__(
         # TODO(b/223262536): Make display_name parameter fully optional in next major release
         display_name: str,
         container_uri: str,
-        command: Sequence[str] = None,
+        command: Optional[Sequence[str]] = None,
         model_serving_container_image_uri: Optional[str] = None,
         model_serving_container_predict_route: Optional[str] = None,
         model_serving_container_health_route: Optional[str] = None,