[ticket/12275] Fix a bug on the event "core.modify_username_string"

Call the event "core.modify_username_string"
also if $mode is "colour", "username" or "profile".

PHPBB3-12275
[ticket/12275] Change if (empty) to if (!isset)

PHPBB3-12275
Merge pull request #2199 from Zoddo/ticket/12275

[ticket/12275] Fix a bug on the event "core.modify_username_string"

* Zoddo/ticket/12275:
  [ticket/12275] Change if (empty) to if (!isset)
  [ticket/12275] Fix a bug on the event "core.modify_username_string"