Add default post text to onboarding flow in web UI (#24705)

app/javascript/mastodon/reducers/compose.js

@@ -528,7 +528,7 @@ export default function compose(state = initialState, action) {
   case COMPOSE_LANGUAGE_CHANGE:
     return state.set('language', action.language);
   case COMPOSE_FOCUS:
-    return state.set('focusDate', new Date());
+    return state.set('focusDate', new Date()).update('text', text => text.length > 0 ? text : action.defaultText);
   default:
     return state;
   }