fix: set job_name to None when initializing TrainingStep

Author: Jaya Kasiraj

src/sagemaker/workflow/steps.py

@@ -464,6 +464,7 @@ def __init__(
         self.step_args = step_args
         self.estimator = estimator
         self.inputs = inputs
+        self.job_name = None
 
         self._properties = Properties(
             step_name=name, step=self, shape_name="DescribeTrainingJobResponse"
@@ -492,6 +493,7 @@ def __init__(
                 DeprecationWarning,
             )
 
+
     @property
     def arguments(self) -> RequestType:
         """The arguments dictionary that is used to call `create_training_job`.