Add missing back button header for invalid account (#12094)

Should fix #6786

app/javascript/mastodon/features/account_timeline/index.js

@@ -83,6 +83,7 @@ class AccountTimeline extends ImmutablePureComponent {
     if (!isAccount) {
       return (
         <Column>
+          <ColumnBackButton multiColumn={multiColumn} />
           <MissingIndicator />
         </Column>
       );