fix type error in web builds (#61556)

packages/flutter/lib/src/foundation/_platform_web.dart

diff --git a/packages/flutter/lib/src/foundation/_platform_web.dart b/packages/flutter/lib/src/foundation/_platform_web.dart
index 42c9052..02b579a 100644
--- a/packages/flutter/lib/src/foundation/_platform_web.dart
+++ b/packages/flutter/lib/src/foundation/_platform_web.dart
@@ -17,7 +17,7 @@
 }
 
 platform.TargetPlatform _browserPlatform() {
-  final String navigatorPlatform = html.window.navigator.platform.toLowerCase();
+  final String navigatorPlatform = html.window.navigator.platform?.toLowerCase() ?? '';
   if (navigatorPlatform.startsWith('mac')) {
     return platform.TargetPlatform.macOS;
   }