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.

src/audio/SDL_audio.c

@@ -972,7 +972,7 @@ SDL_AudioInit(const char *driver_name)
         driver_name = SDL_getenv("SDL_AUDIODRIVER");
     }
 
-    if (driver_name != NULL) {
+    if (driver_name != NULL && *driver_name != 0) {
         const char *driver_attempt = driver_name;
         while (driver_attempt != NULL && *driver_attempt != 0 && !initialized) {
             const char *driver_attempt_end = SDL_strchr(driver_attempt, ',');