[ticket/12025] Post Preview no longer shows warnings

https://tracker.phpbb.com/browse/PHPBB3-12025

PHPBB3-12025
[ticket/12025] Don't show any error on preview if the message is empty

PHPBB3-12025
[ticket/12025] Fixing the condition....

PHPBB3-12025
[ticket/12025] Apply DeMorgan

PHPBB3-12025
Merge pull request #2360 from Nicofuma/ticket/12025

[ticket/12025] Post Preview no longer shows warnings

* Nicofuma/ticket/12025:
  [ticket/12025] Apply DeMorgan
  [ticket/12025] Fixing the condition....
  [ticket/12025] Don't show any error on preview if the message is empty
  [ticket/12025] Post Preview no longer shows warnings