Apply strictNullChecks to src/components/views/voip/* (#10692)

* change interface to discriminated union

* fix ts errors

* use definite assertion operator

* use | undefined, instead of assertion operator

* correct the button press arguments for dial button

* update props to have `Props` suffix

* add comment

* remove unnecessary default

* remove unnecessary comment

* make classname match filename

* setup test file

* export consts for testing

* add initial test

* add another DialPad test

* fix imports

src/components/views/dialogs/InviteDialog.tsx

@@ -1278,7 +1278,7 @@ export default class InviteDialog extends React.PureComponent<Props, IInviteDial
         let title;
         let helpText;
         let buttonText;
-        let goButtonFn;
+        let goButtonFn: (() => Promise<void>) | null = null;
         let consultConnectSection;
         let extraSection;
         let footer;