This was a result of the fix for #20944 optimizing a query to only
include the `id` field with `.only(id)`. Since `Prefix.__init__()`
caches original values from other fields (`_prefix` and `_vrf_id`),
these cached values are `None` at init-time.
This might not normally be a problem, but the sequence of events in
the bug report also end up causing the `handle_prefix_saved` handler
to run, which uses an ORM lookup, (either `net_contained_or_equal`
original`net_contained`) that does not support a query argument of
`None`.