fix: FunctionAgent and ReActAgent error out when LLM response contains no chat messages

[masci]

Description

With this PR we handle the case when an LLM returns no chat messages in the response, propagating the right error and setting a fallback default value. Previously the agent would fail on something like this:

UnboundLocalError: cannot access local variable 'r' where it is not associated with a value

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

[seldo]

LGTM