Add check for undefined UUID
Add check for undefined UUID

This check is needed at there are tons of LDAP implementations out there
and none has at least one guaranteed unique field. As we currently check
three fields and added an option to select one yourself, it's still not
said that any of these fields is set. This will now create an error
and fail the authentication instead of letting people may get access to
other people's notes which are stored under a this way deterministic
wrong userid named `LDAP-undefined`.

Signed-off-by: Sheogorath <1c2647ce2ee8a4ec8e5c8b1b245ca38f94844bab@shivering-isles.com>

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