Allow sending a message with attached files only, such as an image

[marc-farre]

Is your feature request related to a problem? Please describe.

Sometimes, you just want to reply by sharing an image.

But currently, you have this error if you don't write some text:

Message cannot be blank.

Describe the solution you'd like

Allow sending a message with attached files without any text in the message field.

[marc-farre]

@luke- If you agree with the idea, I can send a PR.

[luke-]

@marc-farre Sounds good to me. We have to consider the effects on notifications, though. But that should be fine.

[marc-farre]

@luke- PR #425 (notifications tested)

There is an error in the tests, but not related to this issue:

1. NotificationTest: Notifications are sent after adding participant
   Test codeception/unit/NotificationTest.php:testNotificationsAreSentAfterAddingParticipant
   Failed asserting that two strings are equal.

• Expected | + Actual
  @@ @@
  -'New message from Peter Tester'
  +'You joined the conversation.'

[luke-]

@marc-farre Thanks! Can you please take a look into the test as well?

[marc-farre]

@luke- I first thought it was a test error in the master branch, as it cannot be related to my changes, but as I have no error on my local dev instance, I re-runned the tests on GitHub, and now they pasted, without any extra commit.
So just a GitHub temporary bug.