sending messages to unknown rooms creates redundant state groups

This issue has been migrated from [#3625](https://github.com/matrix-org/synapse/issues/3625).

---

Not sure if this only applies to ASes, but every time the RSS bot tries to send a message to this (unknown) room, we get a new (empty) state group:

```
2018-07-30 17:52:30,048 - synapse.access.http.8103 - 93 - INFO - PUT-3808195 - 2a04:3541:1000:500:6866:a4ff:fe4c:2321 - 8103 - Received request: PUT /_matrix/client/r0/rooms/%21LInBjthlgdanlFFgGB:im.datenpunk.io/send/m.room.message/go1532973150036318470?access_token=
2018-07-30 17:52:30,049 - synapse.util.async - 241 - INFO - PUT-3808195 - Acquired uncontended linearizer lock 'room_event_creation_limit' for key '!LInBjthlgdanlFFgGB:im.datenpunk.io'
2018-07-30 17:52:30,060 - synapse.util.async - 241 - INFO - PUT-3808195 - Acquired uncontended linearizer lock 'state_resolve_lock' for key frozenset([])
2018-07-30 17:52:30,060 - synapse.state - 471 - INFO - PUT-3808195 - Resolving state for !LInBjthlgdanlFFgGB:im.datenpunk.io with 0 groups
2018-07-30 17:52:30,060 - synapse.util.async - 250 - INFO - PUT-3808195 - Releasing linearizer lock 'state_resolve_lock' for key frozenset([])
2018-07-30 17:52:30,065 - synapse.util.async - 250 - INFO - PUT-3808195 - Releasing linearizer lock 'room_event_creation_limit' for key '!LInBjthlgdanlFFgGB:im.datenpunk.io'
2018-07-30 17:52:30,065 - synapse.http.server - 83 - INFO - PUT-3808195 - SynapseError: 403 - Room '!LInBjthlgdanlFFgGB:im.datenpunk.io' does not exist
2018-07-30 17:52:30,065 - synapse.access.http.8103 - 139 - INFO - PUT-3808195 - 2a04:3541:1000:500:6866:a4ff:fe4c:2321 - 8103 - {@_neb_rssbot_=40oma=3aim.datenpunk.io:matrix.org} Processed request: 0.017sec (0.004sec, 0.000sec) (0.009sec/0.003sec/2) 91B 403 "PUT /_matrix/client/r0/rooms/%21LInBjthlgdanlFFgGB:im.datenpunk.io/send/m.room.message/go1532973150036318470?access_token= HTTP/1.1" "Go-http-client/2.0" [0 dbevts]
```