Missing device_one_time_keys_count should not imply zero

Link to problem area: https://spec.matrix.org/v1.8/client-server-api/#e2e-extensions-to-sync

Split off from https://github.com/matrix-org/matrix-spec/issues/1057#

[Quoting](https://github.com/matrix-org/matrix-spec/issues/1057#issuecomment-1122824696) @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.

Start with the E2E extensions to /sync section of the Matrix Client-Server API at the linked specification page, then review the discussion in issue #1057 for context. Update the specification so an omitted device_one_time_keys_count is not treated as zero, and confirm the wording clearly defines the resulting client behavior.

Written by the indexing model from the issue text.