community[patch]: Fix not to cast to str type when `file_path` is None (#20057)

From `langchain_community 0.0.30`, there's a bug that cannot send a
file-like object via `file` parameter instead of `file path` due to
casting the `file_path` to str type even if `file_path` is None.

which means that when I call the `partition_via_api()`, exactly one of
`filename` and `file` must be specified by the following error message.

however, from `langchain_community 0.0.30`, `file_path` is casted into
`str` type even `file_path` is None in `get_elements_from_api()` and got
an error at `exactly_one(filename=filename, file=file)`.

here's an error message
```
---> 51     exactly_one(filename=filename, file=file)
     53     if metadata_filename and file_filename:
     54         raise ValueError(
     55             "Only one of metadata_filename and file_filename is specified. "
     56             "metadata_filename is preferred. file_filename is marked for deprecation.",
     57         )

File /opt/homebrew/lib/python3.11/site-packages/unstructured/partition/common.py:441, in exactly_one(**kwargs)
    439 else:
    440     message = f"{names[0]} must be specified."
--> 441 raise ValueError(message)

ValueError: Exactly one of filename and file must be specified.
```

So, I simply made a change that casting to str type when `file_path` is
not None.

I use `UnstructuredAPIFileLoader` like below.

```
from langchain_community.document_loaders.unstructured import UnstructuredAPIFileLoader

documents: list = UnstructuredAPIFileLoader(
    file_path=None,
    file=file,  # file-like object, io.BytesIO type
    mode='elements',
    url='http://127.0.0.1:8000/general/v0/general',
    content_type='application/pdf',
    metadata_filename='asdf.pdf',
).load_and_split()
```

libs/community/langchain_community/document_loaders/unstructured.py

@@ -213,7 +213,7 @@ def get_elements_from_api(
         from unstructured.partition.api import partition_via_api
 
         return partition_via_api(
-            filename=str(file_path),
+            filename=str(file_path) if file_path is not None else None,
             file=file,
             api_key=api_key,
             api_url=api_url,