Bug: Fallback content is empty string when expected to be undefined

[alexrisch]

Describe the bug

When a content type fallback is set as undefined, Decoded message's fallback is ""

Expected behavior

Should be undefined

Steps to reproduce the bug

test('register and use custom content types', async () => {
  const bob = await Client.createRandom({
    env: 'local',
    codecs: [new NumberCodec()],
  })
  const alice = await Client.createRandom({
    env: 'local',
    // codecs: [new NumberCodec()],
  })
  bob.register(new NumberCodec())
  const bobConvo = await bob.conversations.newConversation(alice.address)
  const aliceConvo = await alice.conversations.newConversation(bob.address)
  await bobConvo.send(12, { contentType: ContentTypeNumber })
  const messages = await aliceConvo.messages()
  assert(messages.length === 1, 'did not get messages')
  const message = messages[0]
  let messageContent = undefined;
  try {
    messageContent = message.content()
  } catch (err) {
    messageContent = message.fallback
    console.log('Setting fallback', typeof message.fallback, {messageContent})
  }
  assert(
    messageContent !== "",
    'did not get content properly: ' + JSON.stringify(messageContent)
  )
  return true
})

class NumberCodec implements JSContentCodec<NumberRef> {
  contentType = ContentTypeNumber
  // a completely absurd way of encoding number values
  encode(content: NumberRef): EncodedContent {
    return {
      type: ContentTypeNumber,
      parameters: {
        test: 'test',
      },
      content: new TextEncoder().encode(JSON.stringify(content)),
    }
  }
  decode(encodedContent: EncodedContent): NumberRef {
    if (encodedContent.parameters.test !== 'test') {
      throw new Error(`parameters should parse ${encodedContent.parameters}`)
    }
    const contentReceived = JSON.parse(
      new TextDecoder().decode(encodedContent.content)
    ) as NumberRef
    return contentReceived
  }
  fallback(content: NumberRef): string | undefined {
    return undefined
  }
}

[nplasterer]

Is this also the case for non-custom content types? I thought you were seeing it for read receipts as well?