Missing device_one_time_keys_count should not imply zero

[Johennes]

Link to problem area: https://spec.matrix.org/v1.2/client-server-api/#extensions-to-sync-1

Split off from #1057

Quoting @neilalexander:

Honestly the specced behaviour of assuming that a missing value means zero is a footgun anyway. A missing value should mean nothing. If the server doesn't support handling OTKs for example, it should be possible to omit the field and expect that a client will do nothing. We should not be encouraging clients to implement magic behaviours.