fix: fix current_time=Now bug for aadd_documents in TimeWeightedRetriever (#5155)

# Same as PR #5045, but for async

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Fixes #4825 

I had forgotten to update the asynchronous counterpart `aadd_documents`
with the bug fix from PR #5045, so this PR also fixes `aadd_documents`
too.

## Who can review?

Community members can review the PR once tests pass. Tag
maintainers/contributors who might be interested:
@dev2049

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langchain/retrievers/time_weighted_retriever.py

@@ -127,7 +127,9 @@ class TimeWeightedVectorStoreRetriever(BaseRetriever, BaseModel):
         self, documents: List[Document], **kwargs: Any
     ) -> List[str]:
         """Add documents to vectorstore."""
-        current_time = kwargs.get("current_time", datetime.datetime.now())
+        current_time = kwargs.get("current_time")
+        if current_time is None:
+            current_time = datetime.datetime.now()
         # Avoid mutating input documents
         dup_docs = [deepcopy(d) for d in documents]
         for i, doc in enumerate(dup_docs):