fix for missing previous_names in conversation

[0rca]

Sometimes, Slack Web API does not return previous_names for conversation, and requests fail with this kind of error:

user error (ServantError (DecodeFailure "Error in $.channels[0]: When parsing the record ChannelConversation of type Web.Slack.Conversation.ChannelConversation the key previous_names was not present." (Response {responseStatusCode = Status {statusCode = 200, statusMessage = "OK"}, responseHeaders = fromList [("date","Tue, 04 Nov 2025 07:54:59 GMT"),("server","Apache"),("vary","Accept-Encoding"),("x-slack-req-id","22d2092ddc4f20a01ec0163979b4d2e7"),("x-content-type-options","nosniff"),("x-xss-protection","0"),("pragma","no-cache"),("cache-control","private, no-cache, no-store, must-revalidate"),("expires","Sat, 26 Jul 1997 05:00:00 GMT"),("content-type","application/json; charset=utf-8"),("x-accepted-oauth-scopes","channels:read,groups:read,mpim:read,im:read,read"),("x-oauth-scopes","chat:write,groups:read,channels:read"),("access-control-expose-headers","x-slack-req-id, retry-after"),("access-control-allow-headers","slack-route, x-slack-version-ts, x-b3-traceid, x-b3-spanid, x-b3-parentspanid, x-b3-sampled, x-b3-flags"),("strict-transport-security","max-age=31536000; includeSubDomains; preload"),("referrer-policy","no-referrer"),("x-slack-unique-id","XXXXXXXXXXXXXXXXXXXXXXXXXXX"),("x-slack-backend","r"),("access-control-allow-origin","*"),("via","1.1 slack-prod.tinyspeck.com, envoy-www-iad-tsmdntwm,envoy-edge-canary-sin-mzuhckfu"),("content-encoding","gzip"),("content-length","381"),("x-envoy-attempt-count","1"),("x-envoy-upstream-service-time","301"),("x-backend","main_normal main_canary_with_overflow main_control_with_overflow"),("x-server","slack-www-hhvm-main-iad-qqux"),("x-slack-shared-secret-outcome","no-match"),("x-edge-backend","envoy-www"),("timing-allow-origin","*"),("alt-svc","h3=\":443\"; ma=10800, h3-29=\":443\"; ma=10800, quic=\":443\"; ma=10800"),("x-slack-edge-shared-secret-outcome","no-match")], responseHttpVersion = HTTP/1.1, responseBody = "{\"ok\":true,\"channels\":[{\"id\":\"C0000000000\",\"created\":1761988686,\"creator\":\"UAAAAAAAA\",\"is_org_shared\":false,\"is_im\":false,\"context_team_id\":\"TAAAAAAAA\",\"updated\":1761988686476,\"name\":\"bot-test\",\"name_normalized\":\"bot-test\",\"is_channel\":true,\"is_group\":false,\"is_mpim\":false,\"is_private\":true,\"is_archived\":false,\"is_general\":false,\"is_shared\":false,\"is_ext_shared\":false,\"unlinked\":0,\"is_pending_ext_shared\":false,\"pending_shared\":[],\"parent_conversation\":null,\"purpose\":{\"value\":\"\",\"creator\":\"\",\"last_set\":0},\"topic\":{\"value\":\"\",\"creator\":\"\",\"last_set\":0},\"shared_team_ids\":[\"TAAAAAAAA\"],\"pending_connected_team_ids\":[],\"is_member\":true,\"num_members\":2}],\"response_metadata\":{\"next_cursor\":\"ZXh0ZXJuYWw6QzA3OEYzODBXVFY=\"}}"})))

This PR goes for minimal diff, but the change is breaking. Another, proper way to fix this would be, in this case, to interpret missing previous_names into empty list, but this would require writing custom JSON parser for Conversation. Tell me what you think.

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