fix: unhandled incoming frames before websocket connection is established

[maslowalex]

This pull request is an attempt to fix currently unhandled scenario when we have incoming frames before the Websocket connection is being established.

The error stacktrace:

[error] 21:52:29.761  :gen_statem #PID<0.629.0> terminating
** (BadMapError) expected a map, got:

    nil

    (mint_web_socket 1.0.4) lib/mint/web_socket/frame.ex:103: Mint.WebSocket.Frame.encode/2
    (fresh 0.4.4) lib/fresh/connection.ex:221: Fresh.Connection.send_frame/2
    (elixir 1.19.2) lib/enum.ex:2520: Enum."-reduce/3-lists^foldl/2-0-"/3
    (fresh 0.4.4) lib/fresh/connection.ex:280: Fresh.Connection.handle_generic_callback/2
    (fresh 0.4.4) lib/fresh/connection.ex:139: Fresh.Connection.connected/3
    (stdlib 7.1) gen_statem.erl:3748: :gen_statem.loop_state_callback/11
    (stdlib 7.1) proc_lib.erl:333: :proc_lib.init_p_do_apply/3
Queue: [info: {:subscribe_candles, ["5m"]}]
Postponed: []