langchain_community: Add default None values to DocumentAttributeValue class properties (#28785)

**Description**: 
This PR addresses an issue where the DocumentAttributeValue class
properties did not have default values of None. By explicitly setting
the Optional attributes (DateValue, LongValue, StringListValue, and
StringValue) to default to None, this change ensures the class functions
as expected when no value is provided for these attributes.

**Changes Made**:
Added default None values to the following properties of the
DocumentAttributeValue class:
DateValue
LongValue
StringListValue
StringValue
Removed the invalid argument extra="allow" from the BaseModel
inheritance.
Dependencies: None.

**Twitter handle (optional)**: @__korikori1021

**Checklist**
- [x] Verified that KendraRetriever works as expected after the changes.

Co-authored-by: y1u0d2a1i <y.kotani@raksul.com>

libs/community/langchain_community/retrievers/kendra.py

@@ -119,13 +119,13 @@ class AdditionalResultAttribute(BaseModel, extra="allow"):  # type: ignore[call-
 class DocumentAttributeValue(BaseModel, extra="allow"):  # type: ignore[call-arg]
     """Value of a document attribute."""
 
-    DateValue: Optional[str]
+    DateValue: Optional[str] = None
     """The date expressed as an ISO 8601 string."""
-    LongValue: Optional[int]
+    LongValue: Optional[int] = None
     """The long value."""
-    StringListValue: Optional[List[str]]
+    StringListValue: Optional[List[str]] = None
     """The string list value."""
-    StringValue: Optional[str]
+    StringValue: Optional[str] = None
     """The string value."""
 
     @property