add condition to avoid error when 'model_dir' is None

[yijiezh]

Issue #, if available:
Tuning job failed with RL container with error trace:

AlgorithmError: framework error: 
Traceback (most recent call last): 
File \"/usr/local/lib/python3.6/dist-packages/sagemaker_containers/_trainer.py\", line 84, in train\n  entrypoint()\n File \"/usr/local/lib/python3.6/dist-packages/sagemaker_tensorflow_container/training.py\", line 206, in main
model_dir = _model_dir_with_training_job(hyperparameters.get('model_dir'), env.job_name)
File \"/usr/local/lib/python3.6/dist-packages/sagemaker_tensorflow_container/training.py\", line 189, in _model_dir_with_training_job\n  if model_dir.startswith('/opt/ml'):
AttributeError: 'NoneType' object has no attribute 'startswith'\n\n'NoneType' object has no attribute 'startswith'

Description of changes:
Add a condition to avoid the above error.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-tensorflow-training-container-pr
• Commit ID: 1224c13
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-tensorflow-training-toolkit-pr
• Commit ID: 1224c13
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository