ARROW-7405: [Java] ListVector isEmpty API is incorrect

Related to [ARROW-7405](https://issues.apache.org/jira/browse/ARROW-7405).

Currently isEmpty API is always return false in BaseRepeatedValueVector, and its subclass ListVector did not overwrite this method.
This will lead to incorrect result, for example, a ListVector with data [1,2], null, [], [5,6] should get [false, false, true, false] with this API, but now it would return [false, false, false, false].

This change implements `isEmpty(int index)` for a `ListVector` that will return `true` if the index is an empty list or a null value, and `false` otherwise.

Closes #6044 from tianchen92/ARROW-7405 and squashes the following commits:

797f9b7 <tianchen> fix style and comments
bbe35db <tianchen> treat null as empty
41e0ed5 <tianchen> resolve some comments
12810e4 <tianchen> ARROW-7405:  ListVector isEmpty API is incorrect

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Bryan Cutler <cutlerb@gmail.com>

java/vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -719,14 +719,30 @@ public Object getObject(int index) {
   /**
    * Check if element at given index is null.
    *
-   * @param index  position of element
+   * @param index position of element
    * @return true if element at given index is null, false otherwise
    */
   @Override
   public boolean isNull(int index) {
     return (isSet(index) == 0);
   }
 
+  /**
+   * Check if element at given index is empty list.
+   * @param index position of element
+   * @return true if element at given index is empty list or NULL, false otherwise
+   */
+  @Override
+  public boolean isEmpty(int index) {
+    if (isNull(index)) {
+      return true;
+    } else {
+      final int start = offsetBuffer.getInt(index * OFFSET_WIDTH);
+      final int end = offsetBuffer.getInt((index + 1) * OFFSET_WIDTH);
+      return start == end;
+    }
+  }
+
   /**
    * Same as {@link #isNull(int)}.
    *

java/vector/src/test/java/org/apache/arrow/vector/TestListVector.java

@@ -950,6 +950,28 @@ public void testGetBufferSizeFor() {
     }
   }
 
+  @Test
+  public void testIsEmpty() {
+    try (final ListVector vector = ListVector.empty("list", allocator)) {
+      UnionListWriter writer = vector.getWriter();
+      writer.allocate();
+
+      // set values [1,2], null, [], [5,6]
+      writeIntValues(writer, new int[] {1, 2});
+      writer.setPosition(2);
+      writeIntValues(writer, new int[] {});
+      writeIntValues(writer, new int[] {5, 6});
+      writer.setValueCount(4);
+
+      assertFalse(vector.isEmpty(0));
+      assertTrue(vector.isNull(1));
+      assertTrue(vector.isEmpty(1));
+      assertFalse(vector.isNull(2));
+      assertTrue(vector.isEmpty(2));
+      assertFalse(vector.isEmpty(3));
+    }
+  }
+
   private void writeIntValues(UnionListWriter writer, int[] values) {
     writer.startList();
     for (int v: values) {