User can post empty comment by using broken bb-link

Steps to reproduce:

1. Open topic
2. In comment editor form press bb-link button "Inset link"
3. In "URL link" field enter test data "1".
4. Press OK button - dialog window disapeares and in comment field appears "[url=1]1[/url]" with selected "1" in the middle.
5. Press "Del" button at a key board - remains "[url=1][/url]"
6. Press "Answer" button

Actual Result:
Creates empty comment

Expected Result:
Error message should appear "may not be empty
Size must be between 2 and 20000"