CORE, REGISTRAR: fixed check for similar users when searchString is null

CORE:

- When searching for users by mail, there is no member_preferredMail, only
  user_preferredMail so use another select to DB for user_attrs & user_attr_values.

REGISTRAR:

- Do not call findUsers() with null searchString
Merge pull request #210 from zlamalp/registrar

CORE, REGISTRAR: fixed check for similar users when searchString is null