Repository: langchain
Issue: Bug in langchain_community/callbacks/tracers/wandb.py:  'NoneType' object has no attribute 'items'
Labels: 🤖:bug

### Checked other resources

- [X] I added a very descriptive title to this issue.
- [X] I searched the LangChain documentation with the integrated search.
- [X] I used the GitHub search to find a similar question and didn't find it.
- [X] I am sure that this is a bug in LangChain rather than my code.
- [X] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

### Example Code

python code
```python


import os
import wandb
from langchain.agents import AgentType, initialize_agent, load_tools
from langchain_openai import OpenAI
from contextvars import ContextVar
from typing import (
    Optional,
)
from langchain_core.tracers.context import register_configure_hook
from langchain_community.callbacks.tracers.wandb import WandbTracer


os.environ["LANGCHAIN_WANDB_TRACING"] = "true"
os.environ["WANDB_PROJECT"] = 'set your wand project'

wandb_tracing_callback_var: ContextVar[Optional[WandbTracer]] = ContextVar(
    "tracing_wandb_callback", default=None
)
register_configure_hook(
    wandb_tracing_callback_var, True, WandbTracer, "LANGCHAIN_WANDB_TRACING"
)

llm = OpenAI(temperature=0)
tools = load_tools(["llm-math"], llm=llm)

agent = initialize_agent(
    tools, llm, agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION, verbose=False
)

agent.run("What is 2 raised to .123243 power?")

wandb.finish()

```

### Error Message and Stack Trace (if applicable)

`wandb: ERROR WARNING: Failed to serialize model: 'NoneType' object has no attribute 'items'`

### Description

I'm trying to use langchain wandb lib and there is a bug in the model: 
`langchain_community/callbacks/tracers/wandb.py` Line: 202 (version langchain-community@0.3.3)

```python

# def transform_run(run: Dict[str, Any]) -> Dict[str, Any]:
#         ...
#        transformed_dict = transform_serialized(run)

#        serialized = transformed_dict.pop("serialized")
       
        for k, v in serialized.items():
#            transformed_dict[k] = v
#       ...
```

where the object `serialized` is None!

### System Info

System Information
------------------
> OS:  Darwin
> OS Version:  Darwin Kernel Version 22.4.0: Mon Mar  6 20:59:28 PST 2023; root:xnu-8796.101.5~3/RELEASE_ARM64_T6000
> Python Version:  3.11.7 (main, Dec  4 2023, 18:10:11) [Clang 15.0.0 (clang-1500.1.0.2.5)]

Package Information
-------------------
> langchain_core: 0.3.15
> langchain: 0.3.7
> langchain_community: 0.3.3
> langsmith: 0.1.132
> langchain_google_vertexai: 2.0.7
> langchain_openai: 0.2.5
> langchain_text_splitters: 0.3.2

Optional packages not installed
-------------------------------
> langgraph
> langserve

Other Dependencies
------------------
> aiohttp: 3.9.5
> anthropic[vertexai]: Installed. No version info available.
> async-timeout: Installed. No version info available.
> dataclasses-json: 0.6.7
> google-cloud-aiplatform: 1.70.0
> google-cloud-storage: 2.18.2
> httpx: 0.27.0
> httpx-sse: 0.4.0
> jsonpatch: 1.33
> langchain-mistralai: Installed. No version info available.
> numpy: 1.26.4
> openai: 1.54.0
> orjson: 3.10.5
> packaging: 23.2
> pydantic: 2.7.4
> pydantic-settings: 2.5.2
> PyYAML: 6.0.1
> requests: 2.32.3
> requests-toolbelt: 1.0.0
> SQLAlchemy: 2.0.30
> tenacity: 8.4.1
> tiktoken: 0.8.0
> typing-extensions: 4.12.2