Don't expect optional keys in `/sync` responses

**Describe the bug**
The [spec for `/sync` responses](https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-sync) says that all keys except `next_batch` are optional, yet in many spots in sytest (illustrated by test failures down below) these keys are implicitly expected in the response.

**Additional context**
This is a blocker for https://github.com/matrix-org/synapse/pull/8336.

[Here](https://buildkite.com/matrix-dot-org/synapse/builds/11929) is a buildkite build that details this problem.

Start by reviewing sytest's /sync response checks and the failures in the linked Buildkite build. Identify the checks that assume optional response keys, then verify that responses containing only next_batch are accepted while the relevant sytest failures are resolved.

Written by the indexing model from the issue text.