android: Don't forbid paths of "" in the APK assets tree.

This would make `SDL_EnumerateDirectory("assets://")` fail.

We check for empty strings at the higher level already, before we strip out
the "assets://" prefix to pass through the tree.

src/core/android/SDL_android.c

@@ -1910,10 +1910,6 @@ static const APKNode *FindAPKNode(const char *constpath)
 {
     //SDL_Log("FindAPKNode('%s') ...", constpath);
 
-    if (*constpath == '\0') {  // don't allow paths of "".
-        return NULL;
-    }
-
     if (SDL_strncmp(constpath, "assets://", 9) == 0) {
         constpath += 9;
     }