[CLNP-6813] Set initial value for initialized and loading (#1337)

[fix]: Set initial value for initialized and loading

iFood 쪽에서 요구한 사항 중 저 플래그 state들의 기본값을 undefined가 아닌 값으로 바꿔달라는 요청이 있어서
작업했습니다.

Fixes [CLNP-6813](https://sendbird.atlassian.net/browse/CLNP-6813)

### Changelogs

- `GroupChannelProvider` has `loading` and `initialized` flag states as
non-`undefined` value

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[CLNP-6813]:
https://sendbird.atlassian.net/browse/CLNP-6813?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: chrisallo

src/modules/GroupChannel/context/GroupChannelProvider.tsx

@@ -39,6 +39,8 @@ const initialState = {
   fetchChannelError: null,
   nicknamesMap: new Map(),
 
+  initialized: false,
+  loading: true,
   messages: [],
   quoteMessage: null,
   animatedMessageId: null,