Cannot retrieve profile when auth/adapter returns an empty value (non null)

• Sometimes adapters can send in a blank String (not null) when there is no valid session/auth
• Such as Shibboleth when `ShibUseHeaders On` (for HTTP headers)
• HTTP header auth adapter sends the value as is
• Regardless of auth system or auth adapter, this is best handled at the API framework end

Here is my PR: https://github.com/xwiki-contrib/xwiki-authenticator-trusted/pull/5