style: add braces to if statement

Author: LinXunFeng

lib/src/common/observer_widget.dart

@@ -460,7 +460,9 @@ class ObserverWidgetState<
     if (isDependObserveCallback) {
       if (onObserve == null &&
           onObserveAll == null &&
-          (innerListeners?.isEmpty ?? true)) return null;
+          (innerListeners?.isEmpty ?? true)) {
+        return null;
+      }
     }
 
     final isHandlingScroll =

lib/src/sliver/sliver_observer_view.dart

@@ -248,7 +248,9 @@ class MixViewObserverState extends ObserverWidgetState<SliverObserverController,
         isForbidObserveViewportCallback ? null : widget.onObserveViewport;
     if (isDependObserveCallback &&
         onObserveViewport == null &&
-        (innerSliverListeners?.isEmpty ?? true)) return null;
+        (innerSliverListeners?.isEmpty ?? true)) {
+      return null;
+    }
 
     final isHandlingScroll =
         widget.sliverController?.innerIsHandlingScroll ?? false;