bug: meta.update() fails when meta = None

[zoltan-fedor]

This was incorrectly filed against the langfuse repo first (see langfuse/langfuse#1775)

Describe the bug

I believe this Commit da264a9 from last week has introduced a bug at https://github.com/langfuse/langfuse-python/blame/main/langfuse/callback/langchain.py#L722 by allowing returning None (edit: it seems the version before the commit has also allowed returning None, so it is interesting why I only see this now), which then fails at https://github.com/langfuse/langfuse-python/blame/main/langfuse/callback/langchain.py#L445, as None doesn't have an update() method

To reproduce

Run a callback with no metadata or tags and you will hit an error:

File "/..../lib/python3.10/site-packages/langfuse/callback/langchain.py", line 443, in on_tool_start
    meta.update(
    └ None

Additional information

No response

[marcklingen]

thanks for your contribution on this 🙏 released in https://github.com/langfuse/langfuse-python/releases/tag/v2.27.0