hangups fails to send message containing only formatting

[tdryer]

Originally reported here: https://github.com/mtomwing/pickups/issues/8

Sending a message interpreted only as formatting (such as **) will fail:

2015-08-26 18:50:35,945 - hangups.client - DEBUG - Sending Protocol Buffer request conversations/sendchatmessage:
request_header {
  client_version {
    client_id: CLIENT_ID_WEB_GMAIL
    build_type: BUILD_TYPE_PRODUCTION_WEB
    major_version: "chat_web_connection_server_20150821.05_p2"
    version_timestamp: 1440526689
  }
  client_identifier {
    resource: "lcsw_hangoutsE5B7F2A4"
    header_id: "ACC9AAA6270E1674"
  }
  language_code: "en"
}
message_content {
}
event_request_header {
  conversation_id {
    id: "UgzqslUwVUSUq1RoXjF4AaABAQ"
  }
  client_generated_id: 4164476472
  expected_otr: OFF_THE_RECORD_STATUS_OFF_THE_RECORD
  delivery_medium {
    medium_type: DELIVERY_MEDIUM_BABEL
  }
  event_type: EVENT_TYPE_REGULAR_CHAT_MESSAGE
}
2015-08-26 18:50:36,103 - hangups.client - DEBUG - Received Protocol Buffer response:
response_header {
  status: RESPONSE_STATUS_INVALID_REQUEST
  error_description: "Populated message content, chat_message_spec, media, external image spec, sticker spec, or location is required"
  debug_url: ""
  request_trace_id: "3785311639567483063"
  current_server_time: 1440640236050000
}