Test case:
public void testString127() throws Exception {
TypeCodec codec = new NullableUnicodeString();
byte[] b = new byte[127];
Arrays.fill(b, (byte)'1');
String expected = new String(b);
byte[] bytes = codec.encode(new StringValue(expected));
final ScalarValue value = codec.decode(new ByteArrayInputStream(bytes));
String actual = value.toString();
Assert.assertEquals(expected, actual);
}

Patch file is attached