[FIX] Broken message fields attachment handling

[tassoevan]

Proposed changes (including videos or screenshots)

Avoids an undefined value to break a rendered attachment.

Issue(s)

Steps to test or reproduce

1. Have a message with the following fields:

{
  ...
  "attachments": [{
    "collapsed": true,
    "title": "Title",
    "fields": [{
        "title": "Title"
    }]
  }]
}

2. Click on expand/collapse button.

Further comments