[Java] Fix the issue when waiting an empty list or a null pointer (#3632)

java/runtime/src/main/java/org/ray/runtime/raylet/RayletClientImpl.java

@@ -50,6 +50,11 @@ public class RayletClientImpl implements RayletClient {
   @Override
   public <T> WaitResult<T> wait(List<RayObject<T>> waitFor, int numReturns, int
       timeoutMs, UniqueId currentTaskId) {
+    Preconditions.checkNotNull(waitFor);
+    if (waitFor.isEmpty()) {
+      return new WaitResult<>(new ArrayList<>(), new ArrayList<>());
+    }
+
     List<UniqueId> ids = new ArrayList<>();
     for (RayObject<T> element : waitFor) {
       ids.add(element.getId());

java/test/src/main/java/org/ray/api/test/WaitTest.java

@@ -1,6 +1,7 @@
 package org.ray.api.test;
 
 import com.google.common.collect.ImmutableList;
+import java.util.ArrayList;
 import java.util.List;
 import org.junit.Assert;
 import org.junit.Test;
@@ -56,4 +57,18 @@ public class WaitTest extends BaseTest {
     RayObject<Object> res = Ray.call(WaitTest::waitInWorker);
     res.get();
   }
+
+  @Test
+  public void testWaitForEmpty() {
+    WaitResult<String> result = Ray.wait(new ArrayList<>());
+    Assert.assertTrue(result.getReady().isEmpty());
+    Assert.assertTrue(result.getUnready().isEmpty());
+
+    try {
+      Ray.wait(null);
+      Assert.fail();
+    } catch (NullPointerException e) {
+      Assert.assertTrue(true);
+    }
+  }
 }