[ticket/12984] Remove blank line when no forum description shown

Add a conditional for forum description and move the <br /> within it.
Currently if there is no forum description but there is
1. a displayed forum moderator and/or
2. a displayed sub-forum
a blank line is added in place of the description

PHPBB3-12984
Merge pull request #2886 from Oyabun1/ticket/12984

[ticket/12984] Remove blank line when no forum description shown

* Oyabun1/ticket/12984:
  [ticket/12984] Remove blank line when no forum description shown