Fix line breaks with URLs in plaintext only messages

[n-peugnet]

This fixes an incorrect rendering behaviour in the timeline and the plaintext mode, introduced by #388, for plain text only messages that combined URLs and newlines.

For example with events of this kind:

{
  "type": "m.room.message",
  "content": {
    "body": "Test message :\n\nhttps://alink.com/that/is/longer\nhttps://ashortlink.fr\n\nEnd of message",
    "msgtype": "m.text"
  }
}

This changes makes sure that there is always only one container and no empty text Entities in the HTML message.

Timeline Before

Timeline After

Plaintext Before

Plaintext After

I am sorry for not catching this bug earlier.

This time I tried to test a lot of edge cases and it behaved properly.