[Docs] Add default constructor for psf example

Author: payniexiao

docs/design/psf_get.md

@@ -41,10 +41,6 @@ public class Sum extends GetFunc {
       this.rowIndex = rowIndex;
     }
 
-    public SumParam() {
-      this(-1, -1);
-    }
-    
     public int getRowIndex() {
       return rowIndex;
     }
@@ -160,6 +156,10 @@ public class Sum extends GetFunc {
   public Sum(SumParam param) {
     super(param);
   }
+  
+  public Sum() {
+    this(null);
+  }
 
   // 获取落在该分区的行分片的元素的和，本方法会在PS端调用
   @Override

docs/design/psf_update.md

@@ -131,6 +131,10 @@ public class Random extends UpdaterFunc {
   public Random(UpdaterParam param) {
     super(param);
   }
+  
+  public Random() {
+    this(null);
+  }
 
   // 分区random操作
   @Override