Remove private message disables submit button (edit.js)

[Ruud68]

Describe the bug
when you edit a post and remove a private message (by selecting all the text and removing it), the submit button is disabled making it 'impossible' to submit the edited message

To Reproduce
Steps to reproduce the behavior:

1. create a new post with a private message
2. submit the post
3. edit the post and delete the private message text
4. the submit button is disabled

Expected behavior
submit button should not be disabled

Actual result
submit button is disabled

Workaround
There are two workarounds:

1. add a space in the private message, now it contains text again and the submit button is enabled again
2. go to tab message and make a change in that text, the submit button is enabled again

System information (please complete the following information)

Joomla version: 5.1.1
Kunena version: 6.3.2

Additional context
Add any other context about the problem here.