fix: ignore empty string reasoning_content in _extract_reasoning_from_additional_kwargs

[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Fixes #36194.

Bug

_extract_reasoning_from_additional_kwargs uses reasoning_content is not None to detect a valid reasoning block. Some providers (e.g. Groq) emit "reasoning_content": "" in additional_kwargs even when no reasoning was produced. This caused an empty ReasoningContentBlock ({"type": "reasoning", "reasoning": ""}) to be prepended to the message content list, which some downstream consumers treat as an error.

Fix

Change the guard from is not None to a truthy check:

# Before
if reasoning_content is not None and isinstance(reasoning_content, str):

# After
if reasoning_content and isinstance(reasoning_content, str):

An empty string is falsy, so it is now skipped. A non-empty string still produces the reasoning block as before.

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