Use constant instead of new instance.

Author: Gary Gregory

src/test/java/org/apache/commons/io/output/NullWriterTest.java

@@ -28,7 +28,7 @@ public class NullWriterTest {
     @Test
     public void testNull() {
         final char[] chars = new char[] { 'A', 'B', 'C' };
-        try (final NullWriter writer = new NullWriter()) {
+        try (final NullWriter writer = NullWriter.NULL_WRITER) {
             writer.write(1);
             writer.write(chars);
             writer.write(chars, 1, 1);