Only call clipboard callbacks with mime types they expect

Also clarified that returning NULL from a callback sends zero length data to the receiver, which should be able to handle that.

Fixes https://github.com/libsdl-org/SDL/issues/9586

test/testclipboard.c

@@ -34,6 +34,7 @@ static const void *ClipboardDataCallback(void *userdata, const char *mime_type,
         *size = icon_png_len;
         return icon_png;
     } else {
+        SDL_Log("Called with unexpected mime type: %s", mime_type);
         return NULL;
     }
 }