loginScreen extension point in PluggableAuthenticationService should allow no video to be provided

In LTS 2016, loginScreen extension point forces people to provide at least one video. The video source can be empty, but in this case the background image will be used as its poster attribute.

=> This prevents Studio configuration from being applied properly, because background image will necessarily cover the whole screen even if people choose the contrary in Studio.

=> loginScreen extension point should allow to provide no video, and in this case the login page should not contain a video tag at all.