community[patch]: refactor the type hint of `file_path` in `UnstructuredAPIFileLoader` class (#18839)

* **Description**: add `None` type for `file_path` along with `str` and
`List[str]` types.
* `file_path`/`filename` arguments in `get_elements_from_api()` and
`partition()` can be `None`, however, there's no `None` type hint for
`file_path` in `UnstructuredAPIFileLoader` and `UnstructuredFileLoader`
currently.
* calling the function with `file_path=None` is no problem, but my IDE
annoys me lol.
* **Issue**: N/A
* **Dependencies**: N/A

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/community/langchain_community/document_loaders/unstructured.py

@@ -156,7 +156,7 @@ class UnstructuredFileLoader(UnstructuredBaseLoader):
 
     def __init__(
         self,
-        file_path: Union[str, List[str], Path, List[Path]],
+        file_path: Union[str, List[str], Path, List[Path], None],
         mode: str = "single",
         **unstructured_kwargs: Any,
     ):
@@ -255,7 +255,7 @@ class UnstructuredAPIFileLoader(UnstructuredFileLoader):
 
     def __init__(
         self,
-        file_path: Union[str, List[str]] = "",
+        file_path: Union[str, List[str], None] = "",
         mode: str = "single",
         url: str = "https://api.unstructured.io/general/v0/general",
         api_key: str = "",