Don't expect optional keys in /sync responses

[ShadowJonathan]

Describe the bug
The spec for /sync responses 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 matrix-org/synapse#8336.

Here is a buildkite build that details this problem.

[ShadowJonathan]

#913 looks relevant to this.

[ShadowJonathan]

Some additional context from Conduit about failing tests, some of these are possible to be fixed by this.

[clokep]

#1040 did some work on this, but from there:

It would partially fix #957, but I haven't gone into rooms and stripped those keys, so there may still be some dependencies there. Like room specific account data, summary, etc.