Fixes error when stop reason missing in response.

[3coins]

Fixes #55, #56.

All integration tests pass.

(base) ➜  aws git:(fix-stop-reason-error) make integration_tests             
poetry run pytest tests/integration_tests/
================================================================== test session starts ==================================================================
platform darwin -- Python 3.9.13, pytest-7.4.4, pluggy-1.5.0
rootdir: /Users/pijain/projects/langchain-aws-dev/langchain-aws/libs/aws
configfile: pyproject.toml
plugins: syrupy-4.6.1, anyio-4.3.0, cov-4.1.0, asyncio-0.23.6
asyncio: mode=auto
collected 34 items                                                                                                                                      

tests/integration_tests/test_compile.py .                                                                                                         [  2%]
tests/integration_tests/chat_models/test_bedrock.py ...................                                                                           [ 58%]
tests/integration_tests/embeddings/test_bedrock_embeddings.py .....s...                                                                           [ 85%]
tests/integration_tests/llms/test_sagemaker_endpoint.py ..                                                                                        [ 91%]
tests/integration_tests/retrievers/test_amazon_kendra_retriever.py .                                                                              [ 94%]
tests/integration_tests/retrievers/test_amazon_knowledgebases_retriever.py ..                                                                     [100%]
====================================================== 33 passed, 1 skipped, 2 warnings in 50.68s =======================================================

[3coins]

@arm-diaz @thekrol01 @supreetkt
Can you review and verify this change fixes the issue. Thanks.

[arm-diaz]

This looks good. Thank you @3coins !

[thekrol01]

Same here, looks good! Great work @3coins !

[supreetkt]

Tried a few models as well. Looks good to me.

[nithiyn]

Thank you!