SemanticsUpdateBuilder.updateNode(): make TextDirection non-nullable

lib/ui/semantics.dart

diff --git a/lib/ui/semantics.dart b/lib/ui/semantics.dart
index e0da396..667d4d1 100644
--- a/lib/ui/semantics.dart
+++ b/lib/ui/semantics.dart
@@ -818,7 +818,7 @@
     required String hint,
     required List<StringAttribute> hintAttributes,
     required String tooltip,
-    required TextDirection? textDirection,
+    required TextDirection textDirection,
     required Float64List transform,
     required Int32List childrenInTraversalOrder,
     required Int32List childrenInHitTestOrder,
@@ -888,7 +888,7 @@
     required String hint,
     required List<StringAttribute> hintAttributes,
     required String tooltip,
-    required TextDirection? textDirection,
+    required TextDirection textDirection,
     required Float64List transform,
     required Int32List childrenInTraversalOrder,
     required Int32List childrenInHitTestOrder,