apacheGH-34610: [Java] Fix valueCount and field name when loading/tra…

…nsferring NullVector (apache#38973)

### Rationale for this change

This is a follow up of apache#34611 (thanks @ sunchao)
We should set `valueCount` when loading `NullVector`. Unfortunately there's no `nullCount` field.

### What changes are included in this PR?

* `valueCount` added to NullVector
* `name` added to NullVector's TransferPair

### Are these changes tested?

Yes. unit tested.

### Are there any user-facing changes?

Yes, a NullVector with a non-null name will maintain the name when transferred via `TransferPair` instead of defaulting to the default name `$data$`.
* Closes: apache#34610

Authored-by: Chao Sun <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

java/c/src/test/java/org/apache/arrow/c/RoundtripTest.java

@@ -476,6 +476,13 @@ public void testUInt8Vector() {
     }
   }
 
+  @Test
+  public void testNullVector() {
+    try (final NullVector vector = new NullVector("v", 1024)) {
+      assertTrue(roundtrip(vector, NullVector.class));
+    }
+  }
+
   @Test
   public void testVarBinaryVector() {
     try (final VarBinaryVector vector = new VarBinaryVector("v", allocator)) {

java/vector/src/main/java/org/apache/arrow/vector/NullVector.java

@@ -57,6 +57,16 @@ public NullVector(String name) {
     this(name, FieldType.nullable(Types.MinorType.NULL.getType()));
   }
 
+  /**
+   * Instantiate a NullVector with the given number of values.
+   *
+   * @param name name of the vector
+   * @param valueCount number of values (i.e., nulls) in this vector.
+   */
+  public NullVector(String name, int valueCount) {
+    this(new Field(name, FieldType.nullable(Types.MinorType.NULL.getType()), null), valueCount);
+  }
+
   /**
    * Instantiate a NullVector.
    *
@@ -73,8 +83,18 @@ public NullVector(String name, FieldType fieldType) {
    * @param field field materialized by this vector.
    */
   public NullVector(Field field) {
-    this.valueCount = 0;
+    this(field, 0);
+  }
+
+  /**
+   * Instantiate a NullVector with the given number of values.
+   *
+   * @param field field materialized by this vector.
+   * @param valueCount number of values (i.e., nulls) in this vector.
+   */
+  public NullVector(Field field, int valueCount) {
     this.field = field;
+    this.valueCount = valueCount;
   }
 
   @Deprecated
@@ -106,7 +126,7 @@ public Types.MinorType getMinorType() {
 
   @Override
   public TransferPair getTransferPair(BufferAllocator allocator) {
-    return getTransferPair((String) null, allocator);
+    return getTransferPair(getName(), allocator);
   }
 
   @Override
@@ -159,12 +179,12 @@ public int getValueCapacity() {
 
   @Override
   public TransferPair getTransferPair(String ref, BufferAllocator allocator) {
-    return new TransferImpl();
+    return new TransferImpl(ref);
   }
 
   @Override
   public TransferPair getTransferPair(Field field, BufferAllocator allocator) {
-    return new TransferImpl();
+    return new TransferImpl(field.getName());
   }
 
   @Override