When GST_DISABLE_GST_DEBUG is defined, the log context functions were
missing their stub implementations, causing link errors. Add the missing
stub functions to the disabled debug section.
Also add (nullable) annotation to gst_log_context_get_category since
it can return NULL when debugging is disabled.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9122>