dummyvideo: Change a thing returning a 0 pointer to return NULL instead.

src/video/dummy/SDL_nullvideo.c

@@ -89,13 +89,13 @@ static SDL_VideoDevice *DUMMY_InternalCreateDevice(const char *enable_hint)
     SDL_VideoDevice *device;
 
     if (!DUMMY_Available(enable_hint)) {
-        return 0;
+        return NULL;
     }
 
     /* Initialize all variables that we clean on shutdown */
     device = (SDL_VideoDevice *)SDL_calloc(1, sizeof(SDL_VideoDevice));
     if (!device) {
-        return 0;
+        return NULL;
     }
     device->is_dummy = SDL_TRUE;