fix(types): Make pictureUrl optional in GroupSummaryResponse

We have a case where the `pictureUrl` is not available. This commit
makes the `pictureUrl` optional in the `GroupSummaryResponse` type.
The documentation for the `GroupSummaryResponse` type has also proven
this to be the case.

See https://developers.line.biz/en/reference/messaging-api/#get-group-summary-response

Author: Gasol

lib/types.ts

@@ -2834,7 +2834,7 @@ export type VerifyIDToken = {
 export type GroupSummaryResponse = {
   groupId: string;
   groupName: string;
-  pictureUrl: string;
+  pictureUrl?: string;
 };
 
 /**