gstreamer: meta: Allow to register custom meta without transform function
The Rust bindings should allow the transform function to be "None". But the current version always expects it to be provided https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/blob/main/gstreamer/src/meta.rs#L537 the docs allow to be NULL https://gstreamer.freedesktop.org/documentation/gstreamer/gstmeta.html?gi-language=c#gst_meta_register_custom
Edited by Rafael Carício