emscripten: Let SDL_HINT_EMSCRIPTEN_KEYBOARD_ELEMENT accept "".

This is used to say "don't even try to listen for keypresses," for apps that
are managing this outside of SDL.

Fixes #10292.

include/SDL3/SDL_hints.h

@@ -730,6 +730,7 @@ extern "C" {
  * - "#document": the javascript document object
  * - "#screen": the javascript window.screen object
  * - "#canvas": the WebGL canvas element
+ * - "": Don't bind anything at all
  * - any other string without a leading # sign applies to the element on the
  *   page with that ID.
  *