[ticket/12331] Fix SQL error when inserting a new entry to profile field data

There was a "Field 'pf_phpbb_interests' doesn't have a default value" error,
because we didn't specify values for the disabled fields.
We can not disable the sql_return_on_error() thou, because it otherwise we run
into "Duplicate entry '2' for key 'PRIMARY'" errors when the user submits the
form again but didn't change any values.

PHPBB3-12331
[ticket/12331] Add functional test for saving custom profile fields

PHPBB3-12331
Merge remote-tracking branch 'github-nickvergessen/ticket/12331' into develop-ascraeus

* github-nickvergessen/ticket/12331:
  [ticket/12331] Add functional test for saving custom profile fields
  [ticket/12331] Fix SQL error when inserting a new entry to profile field data