Fix types so the store can return undefined channels (#26393)

* Fix types so the store can return undefined channels

* Address feedback

* Revert gm change

* Fix lint

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Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/plugins/call_button/call_button.tsx

@@ -20,7 +20,7 @@ import type {PluginComponent} from 'types/store/plugins';
 import './call_button.scss';
 
 type Props = {
-    currentChannel: Channel;
+    currentChannel?: Channel;
     channelMember?: ChannelMembership;
     pluginCallComponents: PluginComponent[];
     sidebarOpen: boolean;