dos: Don't do parameter validation in SDL_SYS_GetPrefPath.

The higher level did it already.

src/filesystem/dos/SDL_sysfilesystem.c

@@ -125,20 +125,11 @@ char *SDL_SYS_GetPrefPath(const char *org, const char *app)
 {
     char *result = NULL;
     size_t len;
-    if (!app) {
-        SDL_InvalidParamError("app");
-        return NULL;
-    }
-
     const char *base = SDL_GetBasePath();
     if (!base) {
         return NULL;
     }
 
-    if (!org) {
-        org = "";
-    }
-
     len = SDL_strlen(base) + SDL_strlen(org) + SDL_strlen(app) + 4;
     result = (char *)SDL_malloc(len);
     if (result) {