Add missing test case from coverage report (#23)

Author: dampcake

src/test/java/com/dampcake/bencode/BencodeOutputStreamTest.java

@@ -92,11 +92,17 @@ public void testWriteStringEmptyByteArray() throws Exception {
     }
 
     @Test
-    public void testWriteStringNullByteArray() throws Exception {
+    public void testWriteStringNullByteBuffer() throws Exception {
         assertThrows(NullPointerException.class, () -> instance.writeString((ByteBuffer) null));
         assertEquals(0, out.toByteArray().length);
     }
 
+    @Test
+    public void testWriteStringNullByteArray() throws Exception {
+        assertThrows(NullPointerException.class, () -> instance.writeString((byte[]) null));
+        assertEquals(0, out.toByteArray().length);
+    }
+
     @Test
     public void testWriteNumber() throws Exception {
         instance.writeNumber(123456);