[ticket/12881] Checks earlier if server-side topic marking is enabled

PHPBB3-12881
Merge pull request #2777 from Nicofuma/ticket/12881

[ticket/12881] Debug error - Undefined index: mark_time

* Nicofuma/ticket/12881:
  [ticket/12881] Checks earlier if server-side topic marking is enabled
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12881] Checks earlier if server-side topic marking is enabled