Avoid unexpected error when stream chat doesn't yield

[joelrorseth]

Description

In some rare cases where no response is yielded by an underlying LLM, stream_chat and astream_chat can throw an unexpected error. This is because, in cases where nothing is yielded, we do not set a variable x that is referenced later in the code. To solve this, I swapped x for last_response in the offending code (the construction of LLMChatEndEvent). Since last_response also may not be set, I changed the type of LLMChatEndEvent.response from ChatResponse to Optional[ChatResponse]. This type change could be a breaking change to users, but I didn't see any other references to this type in this repo.

Fixes #13133

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added new unit/integration tests
• Added new notebook (that tests end-to-end)
• I stared at the code and made sure it makes sense

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods