community: Fix AWS DocumentDB similarity_search when filter is None (#24777)

**Description**

Fixes DocumentDBVectorSearch similarity_search when no filter is used;
it defaults to None but $match does not accept None, so changed default
to empty {} before pipeline is created.

**Issue**

AWS DocumentDB similarity search does not work when no filter is used.
Error msg: "the match filter must be an expression in an object" #24775

**Dependencies**

No dependencies

**Twitter handle**

https://x.com/perepasamonte

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/langchain_community/vectorstores/documentdb.py

@@ -327,6 +327,10 @@ class DocumentDBVectorSearch(VectorStore):
         Returns:
             A list of documents closest to the query vector
         """
+        # $match can't be null, so intializes to {} when None to avoid
+        # "the match filter must be an expression in an object"
+        if not filter:
+            filter = {}
         pipeline: List[dict[str, Any]] = [
             {"$match": filter},
             {