Write `null` for `current-snapshot-id`

[Fokko]

Based on the checks in #742 (comment), it looks like we write -1 when there is no snapshot, which is not correct.

[c-thiel]

@Fokko for this field but also others (properties, snapshots themselves, snapshot_log) should we skip serializing them if they are none or add null?

[Fokko]

The writer-requirements sections of the spec clarifies this:

The current-snapshot-id may be missing too since it is optional in V1. I think it is also fine for properties, snapshots, and snapshot-log to be missing if they are empty, this should all be handled fine by the reader.