Fix Android TitleStorage: avoid leading '/' and handle NULL base path

[corentin35000]

On Android, assets are always packaged inside the assets/ folder of the APK.
There is no need to force a leading / in the path — on the contrary, having "/images/foo.png" makes AAssetManager fail, because it expects relative paths like "images/foo.png".

Previously:

Calling SDL_OpenTitleStorage("") built a base path of "/", producing lookups like "/images/..." that fail on Android.

Calling SDL_OpenTitleStorage(NULL, …) relied on SDL_GetBasePath(), but on Android this always returns NULL, so the call failed with “No available title storage driver”.

This patch fixes both issues by:

Treating override == "" as an empty base path (no leading /).

Falling back to an empty base path when SDL_GetBasePath() returns NULL on Android.

This allows both SDL_OpenTitleStorage("") and SDL_OpenTitleStorage(NULL, …) to work correctly on Android without any extra changes.
On other platforms, behavior remains unchanged.

Tested on macOS, Linux, Windows, and Android — everything works as expected.

Related issues: #13929, #13933

[corentin35000]

let me know if this is okay with you @slouken :)

[icculus]

I'm wondering if it's better to have Android not return NULL from SDL_GetBasePath()...if "assets" is guaranteed to be a real path and we can access files in it with fopen(), I'd rather do that...but if it only works with SDL_IOStream (which does a lot of tapdancing for assets on Android), SDL_GetBasePath()!=NULL is only going to cause different problems.