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

src/video/SDL_clipboard.c

@@ -173,6 +173,11 @@ void *SDL_GetInternalClipboardData(SDL_VideoDevice *_this, const char *mime_type
     void *data = NULL;
 
     if (_this->clipboard_callback) {
+        if (!SDL_HasInternalClipboardData(_this, mime_type)) {
+            // The callback hasn't advertised that it can handle this mime type
+            return NULL;
+        }
+
         const void *provided_data = _this->clipboard_callback(_this->clipboard_userdata, mime_type, size);
         if (provided_data) {
             // Make a copy of it for the caller and guarantee null termination
@@ -180,6 +185,8 @@ void *SDL_GetInternalClipboardData(SDL_VideoDevice *_this, const char *mime_type
             if (data) {
                 SDL_memcpy(data, provided_data, *size);
                 SDL_memset((Uint8 *)data + *size, 0, sizeof(Uint32));
+            } else {
+                *size = 0;
             }
         }
     }