Restore previous behavior of empty SDL_AUDIODRIVER trying all drivers. The recent change to make SDL_AUDIODRIVER support comma-separated lists broke the previous behavior where an SDL_AUDIODRIVER that was empty behaved the same as if it was not set at all. This old behavior was necessary to paper over differences in platforms where SDL_setenv may or may not actually delete the env var if an empty string is specified. This patch just adds a simple check to ensure SDL_AUDIODRIVER is not empty before using it, restoring the old interpretation of the empty var.