Improve CSV reader which can't call .strip() on NoneType (#13079)

Improve CSV reader which can't call .strip() on NoneType if there are
less cells in the row compared to the header

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  - **Description:** 
I have a CSV file as followed

```
headerA,headerB,headerC
v1A,v1B,v1C,
v2A,v2B
v3A,v3B,v3C
```
In this case, row 2 is missing a value, which results in reading a None
type. The strip() method can not be called on None, hence raising. In
this PR I am making the change to only call strip if the value if not
None.

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libs/langchain/langchain/document_loaders/csv_loader.py

@@ -102,7 +102,7 @@ class CSVLoader(BaseLoader):
                     f"Source column '{self.source_column}' not found in CSV file."
                 )
             content = "\n".join(
-                f"{k.strip()}: {v.strip()}"
+                f"{k.strip()}: {v.strip() if v is not None else v}"
                 for k, v in row.items()
                 if k not in self.metadata_columns
             )