videoconvertscale: fix valid_tags NULL-terminated array of strings

The valid_tags array of strings must be NULL-terminated,
as it is passed to the g_strv_contains() function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3603>

subprojects/gst-plugins-base/gst/videoconvertscale/gstvideoconvertscale.c

@@ -686,6 +686,7 @@ gst_video_convert_scale_transform_meta (GstBaseTransform * trans,
     GST_META_TAG_VIDEO_STR,
     GST_META_TAG_VIDEO_ORIENTATION_STR,
     GST_META_TAG_VIDEO_SIZE_STR,
+    NULL
   };
 
   tags = gst_meta_api_type_get_tags (info->api);