Fix `null` signUp URL in various places (#25190)

app/javascript/mastodon/features/interaction_modal/index.jsx

@@ -13,7 +13,7 @@ import { registrationsOpen } from 'mastodon/initial_state';
 
 const mapStateToProps = (state, { accountId }) => ({
   displayNameHtml: state.getIn(['accounts', accountId, 'display_name_html']),
-  signupUrl: state.getIn(['server', 'server', 'registrations', 'url'], '/auth/sign_up'),
+  signupUrl: state.getIn(['server', 'server', 'registrations', 'url'], null) || '/auth/sign_up',
 });
 
 const mapDispatchToProps = (dispatch) => ({

app/javascript/mastodon/features/ui/components/sign_in_banner.jsx

@@ -17,7 +17,7 @@ const SignInBanner = () => {
 
   let signupButton;
 
-  const signupUrl = useAppSelector((state) => state.getIn(['server', 'server', 'registrations', 'url'], '/auth/sign_up'));
+  const signupUrl = useAppSelector((state) => state.getIn(['server', 'server', 'registrations', 'url'], null) || '/auth/sign_up');
 
   if (registrationsOpen) {
     signupButton = (