fix:Add check for empty search results to prevent array out of bounds

Author: SYaoJun

geaflow/geaflow-plugins/geaflow-store/geaflow-store-vector/src/main/java/org/apache/geaflow/store/lucene/GraphVectorIndex.java

@@ -145,6 +145,10 @@ public K searchVectorIndex(boolean isVertex, String fieldName, float[] vector, i
             // Execute search
             TopDocs topDocs = searcher.search(knnQuery, topK);
 
+            if (topDocs.scoreDocs.length == 0) {
+                return null;
+            }
+
             Document firstDoc = searcher.doc(topDocs.scoreDocs[0].doc);
 
             K result;

geaflow/geaflow-plugins/geaflow-store/geaflow-store-vector/src/test/java/org/apache/geaflow/store/lucene/GraphVectorIndexTest.java

@@ -155,4 +155,11 @@ public void testMultipleVectorsSameKeyDifferentFields() {
         assertEquals(result1, key);
         assertEquals(result2, key);
     }
+    @Test
+    public void testSearchNonExistentFieldReturnsNull() {
+        stringIndex.addVectorIndex(true, "vertex_1", "embedding", new float[]{0.1f, 0.2f, 0.3f, 0.4f});
+        // searching for a non-existent field should return null
+        String result = stringIndex.searchVectorIndex(true, "nonexistent_field", new float[]{0.1f, 0.2f, 0.3f, 0.4f}, 1);
+        assertEquals(result, null);
+    }
 }