Don't force displayname if backend already provides one

`\OC_User::loginWithApache` is used in combination with backend mechanisms like our SSO / SAML integration. Those can optionally already provide a displayname using other means. For example by mapping SAML attributes.

The current approach makes it however impossible for backends using `\OCP\Authentication\IApacheBackend` to set a displayname on their own. Because the display name will simply be overwritten with the loginname.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>