fix: Make BaseMemoryService nullable in Runner

This change allows the Runner to be constructed and used even if a BaseMemoryService is not provided, which is the default as of now.

PiperOrigin-RevId: 795476217

Author: google-genai-bot

core/src/main/java/com/google/adk/runner/Runner.java

@@ -48,22 +48,23 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /** The main class for the GenAI Agents runner. */
 public class Runner {
   private final BaseAgent agent;
   private final String appName;
   private final BaseArtifactService artifactService;
   private final BaseSessionService sessionService;
-  private final BaseMemoryService memoryService;
+  private final @Nullable BaseMemoryService memoryService;
 
   /** Creates a new {@code Runner}. */
   public Runner(
       BaseAgent agent,
       String appName,
       BaseArtifactService artifactService,
       BaseSessionService sessionService,
-      BaseMemoryService memoryService) {
+      @Nullable BaseMemoryService memoryService) {
     this.agent = agent;
     this.appName = appName;
     this.artifactService = artifactService;
@@ -103,7 +104,7 @@ public BaseSessionService sessionService() {
     return this.sessionService;
   }
 
-  public BaseMemoryService memoryService() {
+  public @Nullable BaseMemoryService memoryService() {
     return this.memoryService;
   }