When sending a toot, ensure a CW is only set if the CW field is visible (#11206)

In some occasions, such as the browser or a browser extension auto-filling
the existing but disabled/hidden CW field, a CW can be set without the user
knowing.

app/javascript/mastodon/reducers/compose.js

@@ -224,6 +224,7 @@ export default function compose(state = initialState, action) {
       }
     });
   case COMPOSE_SPOILER_TEXT_CHANGE:
+    if (!state.get('spoiler')) return state;
     return state
       .set('spoiler_text', action.text)
       .set('idempotencyKey', uuid());