Fix appservice EDUs failing to send if the EDU doesn't have a room ID

[turt2live]

As is in the case of presence.

Fixes this error:

element-dev-stack-eledev-synapse-1      | 2022-07-09 02:04:50,721 - synapse.metrics.background_process_metrics - 242 - ERROR - as-sender-encrypted_appservices-13 - Background process 'as-sender-encrypted_appservices' threw an exception
element-dev-stack-eledev-synapse-1      | Traceback (most recent call last):
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/twisted/internet/defer.py", line 1660, in _inlineCallbacks
element-dev-stack-eledev-synapse-1      |     result = current_context.run(gen.send, result)
element-dev-stack-eledev-synapse-1      | StopIteration: 504208
element-dev-stack-eledev-synapse-1      | 
element-dev-stack-eledev-synapse-1      | During handling of the above exception, another exception occurred:
element-dev-stack-eledev-synapse-1      | 
element-dev-stack-eledev-synapse-1      | Traceback (most recent call last):
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/metrics/background_process_metrics.py", line 240, in run
element-dev-stack-eledev-synapse-1      |     return await func(*args, **kwargs)
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 274, in _send_request
element-dev-stack-eledev-synapse-1      |     ) = await self._compute_msc3202_otk_counts_and_fallback_keys(
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 321, in _compute_msc3202_otk_counts_and_fallback_keys
element-dev-stack-eledev-synapse-1      |     rooms_of_interesting_users.update(
element-dev-stack-eledev-synapse-1      |   File "/usr/local/lib/python3.9/site-packages/synapse/appservice/scheduler.py", line 322, in <genexpr>
element-dev-stack-eledev-synapse-1      |     ephemeral["room_id"] for ephemeral in ephemerals
element-dev-stack-eledev-synapse-1      | KeyError: 'room_id'

Pull Request Checklist

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[squahtx]

I think this bug has existed since the implementation of MSC3202 in #11617.

[squahtx]

lgtm

[DMRobertson]

Complement failures look like known flakes. Going to manually merge.