langchain[patch]: Assign appropriate default for Optional/Any types (#26325)

This PR was autogenerated using gritql

```
engine marzano(0.1)
language python

class_definition(name=$C, $body, superclasses=$S) where {    
    $C <: ! "Config", // Does not work in this scope, but works after class_definition
    $body <: block($statements),
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        or {
            $y <: `Field($z)`,
            $x <: "model_config"
        }
    },
    // And has either Any or Optional fields without a default
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        $t <: or {
            r"Optional.*",
            r"Any",
            r"Union[None, .*]",
            r"Union[.*, None, .*]",
            r"Union[.*, None]",
        },
        $y <: ., // Match empty node        
        $t => `$t = None`,
    },    
}
```


```shell

grit apply 'class_definition(name=$C, $body, superclasses=$S) where {    
    $C <: ! "Config", // Does not work in this scope, but works after class_definition
    $body <: block($statements),
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        or {
            $y <: `Field($z)`,
            $x <: "model_config"
        }
    },
    // And has either Any or Optional fields without a default
    $statements <: some bubble assignment(left=$x, right=$y, type=$t) as $A where {
        $t <: or {
            r"Optional.*",
            r"Any",
            r"Union[None, .*]",
            r"Union[.*, None, .*]",
            r"Union[.*, None]",
        },
        $y <: ., // Match empty node        
        $t => `$t = None`,
    },    
}
' --language python .

```

libs/langchain/langchain/chains/api/base.py

@@ -198,7 +198,7 @@ try:
         api_docs: str
         question_key: str = "question"  #: :meta private:
         output_key: str = "output"  #: :meta private:
-        limit_to_domains: Optional[Sequence[str]]
+        limit_to_domains: Optional[Sequence[str]] = Field(default_factory=list)
         """Use to limit the domains that can be accessed by the API chain.
         
         * For example, to limit to just the domain `https://www.example.com`, set

libs/langchain/langchain/chains/conversational_retrieval/base.py

@@ -92,7 +92,7 @@ class BaseConversationalRetrievalChain(Chain):
     get_chat_history: Optional[Callable[[List[CHAT_TURN_TYPE]], str]] = None
     """An optional function to get a string of the chat history.
     If None is provided, will use a default."""
-    response_if_no_docs_found: Optional[str]
+    response_if_no_docs_found: Optional[str] = None
     """If specified, the chain will return a fixed response if no docs 
     are found for the question. """
 

libs/langchain/langchain/chains/elasticsearch_database/base.py

@@ -40,7 +40,7 @@ class ElasticsearchDatabaseChain(Chain):
     """Chain for creating the ES query."""
     answer_chain: Runnable
     """Chain for answering the user question."""
-    database: Any
+    database: Any = None
     """Elasticsearch database to connect to of type elasticsearch.Elasticsearch."""
     top_k: int = 10
     """Number of results to return from the query"""

libs/langchain/langchain/chains/moderation.py

@@ -28,8 +28,8 @@ class OpenAIModerationChain(Chain):
             moderation = OpenAIModerationChain()
     """
 
-    client: Any  #: :meta private:
+    client: Any = None  #: :meta private:
-    async_client: Any  #: :meta private:
+    async_client: Any = None  #: :meta private:
     model_name: Optional[str] = None
     """Moderation model name to use."""
     error: bool = False

libs/langchain/langchain/retrievers/document_compressors/embeddings_filter.py

@@ -36,7 +36,7 @@ class EmbeddingsFilter(BaseDocumentCompressor):
     k: Optional[int] = 20
     """The number of relevant documents to return. Can be set to None, in which case
     `similarity_threshold` must be specified. Defaults to 20."""
-    similarity_threshold: Optional[float]
+    similarity_threshold: Optional[float] = None
     """Threshold for determining when two documents are similar enough
     to be considered redundant. Defaults to None, must be specified if `k` is set
     to None."""