[Java] Fix bug of BaseID in multi-threading case. (#4974)

java/api/src/main/java/org/ray/api/id/BaseId.java

@@ -41,13 +41,14 @@ public abstract class BaseId implements Serializable {
    */
   public boolean isNil() {
     if (isNilCache == null) {
-      isNilCache = true;
+      boolean localIsNil = true;
       for (int i = 0; i < size(); ++i) {
         if (id[i] != (byte) 0xff) {
-          isNilCache = false;
+          localIsNil = false;
           break;
         }
       }
+    isNilCache = localIsNil;
     }
     return isNilCache;
   }