Follow up to #39113

lib/ui/platform_dispatcher.dart

@@ -1943,7 +1943,7 @@ class Locale {
   const Locale(
     this._languageCode, [
     this._countryCode,
-  ]) : assert(_languageCode != ''),
+  ]) : assert(_languageCode.isNotEmpty),
        scriptCode = null;
 
   /// Creates a new Locale object.
@@ -1970,10 +1970,10 @@ class Locale {
     String languageCode = 'und',
     this.scriptCode,
     String? countryCode,
-  }) : assert(languageCode != ''),
+  }) : assert(languageCode.isNotEmpty),
        _languageCode = languageCode,
-       assert(scriptCode != ''),
-       assert(countryCode != ''),
+       assert(scriptCode == null || scriptCode.isNotEmpty),
+       assert(countryCode == null || countryCode.isNotEmpty),
        _countryCode = countryCode;
 
   /// The primary language subtag for the locale.

lib/web_ui/lib/platform_dispatcher.dart

@@ -443,17 +443,17 @@ class Locale {
   const Locale(
     this._languageCode, [
     this._countryCode,
-  ])  : assert(_languageCode != ''),
+  ])  : assert(_languageCode.isNotEmpty),
         scriptCode = null;
 
   const Locale.fromSubtags({
     String languageCode = 'und',
     this.scriptCode,
     String? countryCode,
-  })  : assert(languageCode != ''),
+  })  : assert(languageCode.isNotEmpty),
         _languageCode = languageCode,
-        assert(scriptCode != ''),
-        assert(countryCode != ''),
+        assert(scriptCode == null || scriptCode.isNotEmpty),
+        assert(countryCode == null || countryCode.isNotEmpty),
         _countryCode = countryCode;
 
   String get languageCode => _deprecatedLanguageSubtagMap[_languageCode] ?? _languageCode;