Fix NPE when metadata is null in MilvusVectorStore (#4116)

Adds a null check before injecting the distance property into metadata to prevent a NullPointerException
if metadata is missing from the row record. 

The change is intentionally minimal and does not alter existing behavior beyond avoiding the crash.

Some search results do not carry metadata (or the metadata value is null). 
Calling JsonObject.addProperty(...) on a null reference throws an NPE and breaks the mapping pipeline.

Add a null guard around the distance injection step to ensure we do not call addProperty(...) on a null metadata.

Fixes #4115
Auto-cherry-pick to 1.0.x

Signed-off-by: little_huang <53588889+little-huang@users.noreply.github.com>

Author: little-huang

vector-stores/spring-ai-milvus-store/src/main/java/org/springframework/ai/vectorstore/milvus/MilvusVectorStore.java

@@ -379,8 +379,10 @@ public List<Document> doSimilaritySearch(SearchRequest request) {
 				JsonObject metadata = new JsonObject();
 				try {
 					metadata = (JsonObject) rowRecord.get(this.metadataFieldName);
-					// inject the distance into the metadata.
-					metadata.addProperty(DocumentMetadata.DISTANCE.value(), 1 - getResultSimilarity(rowRecord));
+					if (metadata != null) {
+						// inject the distance into the metadata.
+						metadata.addProperty(DocumentMetadata.DISTANCE.value(), 1 - getResultSimilarity(rowRecord));
+					}
 				}
 				catch (ParamException e) {
 					// skip the ParamException if metadata doesn't exist for the custom