fix: guard missing sessions list

Prevent NPE when Vertex AI listSessions response omits or nulls the
`sessions` field.

Adds coverage for missing and null `sessions` responses.

Author: rolandkakonyi

core/src/main/java/com/google/adk/sessions/VertexAiSessionService.java

@@ -119,15 +119,20 @@ public Single<ListSessionsResponse> listSessions(String appName, String userId)
         .map(
             listSessionsResponseMap ->
                 parseListSessionsResponse(listSessionsResponseMap, appName, userId))
-        .defaultIfEmpty(ListSessionsResponse.builder().build());
+        .defaultIfEmpty(ListSessionsResponse.builder().sessions(new ArrayList<>()).build());
   }
 
   private ListSessionsResponse parseListSessionsResponse(
       JsonNode listSessionsResponseMap, String appName, String userId) {
+    JsonNode sessionsNode = listSessionsResponseMap.get("sessions");
+    if (sessionsNode == null || sessionsNode.isNull() || sessionsNode.isEmpty()) {
+      return ListSessionsResponse.builder().sessions(new ArrayList<>()).build();
+    }
     List<Map<String, Object>> apiSessions =
-        objectMapper.convertValue(
-            listSessionsResponseMap.get("sessions"),
-            new TypeReference<List<Map<String, Object>>>() {});
+        objectMapper.convertValue(sessionsNode, new TypeReference<List<Map<String, Object>>>() {});
+    if (apiSessions == null) {
+      apiSessions = new ArrayList<>();
+    }
 
     List<Session> sessions = new ArrayList<>();
     for (Map<String, Object> apiSession : apiSessions) {

core/src/test/java/com/google/adk/sessions/VertexAiSessionServiceTest.java

@@ -25,6 +25,8 @@
 import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import okhttp3.MediaType;
+import okhttp3.ResponseBody;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -37,6 +39,8 @@
 public class VertexAiSessionServiceTest {
 
   private static final ObjectMapper mapper = JsonBaseModel.getMapper();
+  private static final MediaType JSON_MEDIA_TYPE =
+      MediaType.parse("application/json; charset=utf-8");
 
   private static final String MOCK_SESSION_STRING_1 =
       """
@@ -322,6 +326,42 @@ public void listSessions_empty() {
         .isEmpty();
   }
 
+  @Test
+  public void listSessions_missingSessionsField_returnsEmpty() {
+    when(mockApiClient.request("GET", "reasoningEngines/123/sessions?filter=user_id=userX", ""))
+        .thenReturn(
+            new ApiResponse() {
+              @Override
+              public ResponseBody getResponseBody() {
+                return ResponseBody.create(JSON_MEDIA_TYPE, "{}");
+              }
+
+              @Override
+              public void close() {}
+            });
+
+    assertThat(vertexAiSessionService.listSessions("123", "userX").blockingGet().sessions())
+        .isEmpty();
+  }
+
+  @Test
+  public void listSessions_nullSessionsField_returnsEmpty() {
+    when(mockApiClient.request("GET", "reasoningEngines/123/sessions?filter=user_id=userY", ""))
+        .thenReturn(
+            new ApiResponse() {
+              @Override
+              public ResponseBody getResponseBody() {
+                return ResponseBody.create(JSON_MEDIA_TYPE, "{\"sessions\": null}");
+              }
+
+              @Override
+              public void close() {}
+            });
+
+    assertThat(vertexAiSessionService.listSessions("123", "userY").blockingGet().sessions())
+        .isEmpty();
+  }
+
   @Test
   public void listEvents_empty() {
     assertThat(vertexAiSessionService.listEvents("789", "user1", "3").blockingGet().events())