Amazon Bedrock AgentCore Samples - [Bug] 01-tutorials/utils.py throws MalformedPolicyDocument

**Describe the bug**

`create_agentcore_role` in "01-tutorials/utils.py" errors out with:
```text
An error occurred (MalformedPolicyDocument) when calling the PutRolePolicy operation: The policy failed legacy parsing
```
Root cause is that ARN's like this are produced "arn:aws:logs:None:1111111111:log-group:*"

**To Reproduce**

Run "01-tutorials/01-AgentCore-runtime/01-hosting-agent/01-strands-with-bedrock-model/runtime_with_strands_and_bedrock_models.ipynb" with only AWS_REGION set, but **no** AWS_DEFAULT_REGION set

e.g. run the below python code with

```sh
AWS_REGION=us-west-1 python test.py
```

```python
from boto3.session import Session

if __name__ == "__main__":
boto_session = Session()
region = boto_session.region_name
print(region)
```
reproduces this error (no AWS_DEFAULT_REGION set) ie. prints 'None'

**Expected behavior**

Default to AWS_REGION, and if this is not set default to a pre-set region (e.g. us-east-1)

Start in 01-tutorials/utils.py at create_agentcore_role, then reproduce the region lookup with AWS_REGION set and AWS_DEFAULT_REGION unset. Use the referenced runtime_with_strands_and_bedrock_models.ipynb to verify the change; done means the policy no longer contains a None region and the sample runs with the documented fallback behavior.

Written by the indexing model from the issue text.