All other path operations in the code base already use u8path to ensure
that the strings provided by libobs (which will either be ASCII or
UTF-8 byte sequences) are correctly converted into UTF-16 strings.

Changes:
 - Fix possible crashes if config path has unicode characters

When OBS Studio is started in Studio Mode, if a scene collection was not
successfully found and loaded, the OBS Frontend API would, prior to
obsproject/obs-studio@577e350,
emit OBS_FRONTEND_EVENT_SCENE_CHANGED and
OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED anyway. This would result in
currentScene here being a nullptr, causing a crash.

For completeness, check for nullptr here so that we cannot crash if
obs_frontend_get_current_scene() returns a nullptr.

(cherry picked from commit 05e13b1)

Changes:
 - Fix possible crash if OBS started in Studio Mode