Refactor QdrantObjectFactory to allow null metadata values

Signed-off-by: WOONBE <kepull2918@naver.com>

Author: WOONBE

vector-stores/spring-ai-qdrant-store/src/main/java/org/springframework/ai/vectorstore/qdrant/QdrantObjectFactory.java

@@ -16,6 +16,7 @@
 
 package org.springframework.ai.vectorstore.qdrant;
 
+import java.util.HashMap;
 import java.util.Map;
 import java.util.stream.Collectors;
 
@@ -30,6 +31,7 @@
  * Utility methods for building Java objects from io.qdrant.client.grpc.JsonWithInt.Value.
  *
  * @author Anush Shetty
+ * @author Heonwoo Kim
  * @since 0.8.1
  */
 final class QdrantObjectFactory {
@@ -41,7 +43,11 @@ private QdrantObjectFactory() {
 
 	public static Map<String, Object> toObjectMap(Map<String, Value> payload) {
 		Assert.notNull(payload, "Payload map must not be null");
-		return payload.entrySet().stream().collect(Collectors.toMap(e -> e.getKey(), e -> object(e.getValue())));
+		Map<String, Object> map = new HashMap<>();
+		for (Map.Entry<String, Value> entry : payload.entrySet()) {
+			map.put(entry.getKey(), object(entry.getValue()));
+		}
+		return map;
 	}
 
 	private static Object object(ListValue listValue) {