langchain[patch]: Use map() and avoid "ValueError: max() arg is an empty sequence" in MergerRetriever (#18679)

- **Issue:** When passing an empty list to MergerRetriever it fails with
error: ValueError: max() arg is an empty sequence

- **Description:** We have a use case where we dynamically select
retrievers and use MergerRetriever for merging the output of the
retrievers. We faced this issue when the retriever_docs list is empty.
Adding a default 0 for cases when retriever_docs is an empty list to
avoid "ValueError: max() arg is an empty sequence". Also, changed to use
map() which is more than twice as fast compared to the current
implementation.
```
import timeit
# Sample retriever_docs with varying lengths of sublists
retriever_docs = [[i for i in range(j)] for j in range(1, 1000)]
# First code snippet
code1 = '''
max_docs = max(len(docs) for docs in retriever_docs)
'''
# Second code snippet
code2 = '''
max_docs = max(map(len, retriever_docs), default=0)
'''
# Benchmarking
time1 = timeit.timeit(stmt=code1, globals=globals(), number=10000)
time2 = timeit.timeit(stmt=code2, globals=globals(), number=10000)
# Output
print(f"Execution time for code snippet 1: {time1} seconds")
print(f"Execution time for code snippet 2: {time2} seconds")
```

- **Dependencies:** none

libs/langchain/langchain/retrievers/merger_retriever.py

@@ -80,7 +80,7 @@ class MergerRetriever(BaseRetriever):
 
         # Merge the results of the retrievers.
         merged_documents = []
-        max_docs = max(len(docs) for docs in retriever_docs)
+        max_docs = max(map(len, retriever_docs), default=0)
         for i in range(max_docs):
             for retriever, doc in zip(self.retrievers, retriever_docs):
                 if i < len(doc):
@@ -113,7 +113,7 @@ class MergerRetriever(BaseRetriever):
 
         # Merge the results of the retrievers.
         merged_documents = []
-        max_docs = max(len(docs) for docs in retriever_docs)
+        max_docs = max(map(len, retriever_docs), default=0)
         for i in range(max_docs):
             for retriever, doc in zip(self.retrievers, retriever_docs):
                 if i < len(doc):