Allow serializing null properties for Bedrock (#3607)

Fixes #3600

Author: jmartisk

langchain4j-bedrock/src/main/java/dev/langchain4j/model/bedrock/AwsDocumentConverter.java

@@ -23,8 +23,7 @@ class AwsDocumentConverter {
 
     static final ObjectMapper OBJECT_MAPPER = new ObjectMapper()
             .disable(INDENT_OUTPUT)
-            .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
-            .setSerializationInclusion(JsonInclude.Include.NON_NULL);
+            .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
 
     private AwsDocumentConverter() {}
 
@@ -55,6 +54,8 @@ private static Object documentToObject(Document doc) {
             Map<String, Object> innerObject = new HashMap<>();
             doc.asMap().forEach((k, v) -> innerObject.put(k, documentToObject(v)));
             return innerObject;
+        } else if (doc.isNull()) {
+            return null;
         } else {
             return doc.asString();
         }

langchain4j-bedrock/src/test/java/dev/langchain4j/model/bedrock/AwsDocumentConverterTest.java

@@ -22,6 +22,7 @@ void convert_primitive_types_to_json() {
         docMap.put("integer", Document.fromNumber(42));
         docMap.put("double", Document.fromNumber(42.5));
         docMap.put("boolean", Document.fromBoolean(true));
+        docMap.put("null", Document.fromNull());
         Document document = Document.fromMap(docMap);
 
         // When
@@ -32,7 +33,8 @@ void convert_primitive_types_to_json() {
                 .containsIgnoringWhitespaces("\"string\": \"test\"")
                 .containsIgnoringWhitespaces("\"integer\": 42")
                 .containsIgnoringWhitespaces("\"double\": 42.5")
-                .containsIgnoringWhitespaces("\"boolean\": true");
+                .containsIgnoringWhitespaces("\"boolean\": true")
+                .containsIgnoringWhitespaces("\"null\": null");
     }
 
     @Test