GET /room_keys/keys?version=$v returns an error if the backup is empty

[manuroe]

Discussed at https://matrix.to/#/!uewiilduiDRfPomIha:matrix.org/$15398763772905238xioNB:matrix.org.

Instead returning an empty object, we get this error:

{
    errcode = "M_UNKNOWN";
    error = "No room_keys found";
}

[uhoreg]

It looks like the error is set at https://github.com/matrix-org/synapse/pull/4019/files#diff-5c190c6b2c1bbe2f359c3e41932da2dbR71

I think it makes sense to send an error for /room_keys/keys/${roomId}/${sessionId} if no backup is found (though the errcode = "M_UNKNOWN" should be fixed), but for /room_keys/keys/${roomId} and /room_keys/keys, I think it should just send {"sessions": {}} or {"rooms": {}}.

[richvdh]

fixed by #4123