Trim failed when using TrimmedBlobKind to encode all zero string

[libotony]

The following test failed:

    it('trimmedBlobKind encode with all zero string', () => {
      const kind = new RLP.TrimmedBlobKind(4)
      expect(kind.encode('0x00000000', '').toString('hex')).equal('')
    })

[libotony]

fixed in e6231b4