Fix missing None-check in get_conversations

[raymyers]

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• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

End-user friendly description of the problem this fixes or functionality this introduces.

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Summarize what the PR does, explaining any non-trivial design decisions.

Handle the case were get_conversation gets None from attach_conversation, it should be an HTTP 404, currently it's passing it into detach_conversation causing 500s and this runtime exception:

openhands/server/conversation_manager/standalone_conversation_manager.py", line 136, in detach_from_conversation
    sid = conversation.sid
          ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'sid'

Also added a missing await on call to an async conversation_manager function.

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Link of any specific issues this addresses:

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:02987b1-nikolaik   --name openhands-app-02987b1   docker.all-hands.dev/all-hands-ai/openhands:02987b1

[tofarr]

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