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

include/SDL3/SDL_clipboard.h

@@ -198,11 +198,7 @@ extern SDL_DECLSPEC bool SDLCALL SDL_HasPrimarySelectionText(void);
  * \param mime_type the requested mime-type.
  * \param size a pointer filled in with the length of the returned data.
  * \returns a pointer to the data for the provided mime-type. Returning NULL
- *          or setting the length to 0 will cause no data to be sent to the
- *          "receiver". It is up to the receiver to handle this. Essentially
- *          returning no data is more or less undefined behavior and may cause
- *          breakage in receiving applications. The returned data will not be
- *          freed, so it needs to be retained and dealt with internally.
+ *          or setting the length to 0 will cause zero length data to be sent to the "receiver", which should be able to handle this. The returned data will not be freed, so it needs to be retained and dealt with internally.
  *
  * \since This function is available since SDL 3.2.0.
  *

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;
             }
         }
     }

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;
     }
 }