SemanticsUpdateBuilder: make all args non-null (flutter/engine#49148)

This PR is inspired by [the suggestion made here](https://github.com/flutter/engine/pull/47961#discussion_r1406581284).

engine/src/flutter/lib/ui/fixtures/ui_test.dart

@@ -173,6 +173,8 @@ void sendSemanticsUpdate() {
     ),
   ];
 
+  String tooltip = "tooltip";
+
   final Float64List transform = Float64List(16);
   final Int32List childrenInTraversalOrder = Int32List(0);
   final Int32List childrenInHitTestOrder = Int32List(0);
@@ -224,6 +226,7 @@ void sendSemanticsUpdate() {
     decreasedValueAttributes: decreasedValueAttributes,
     hint: hint,
     hintAttributes: hintAttributes,
+    tooltip: tooltip,
     textDirection: TextDirection.ltr,
     transform: transform,
     childrenInTraversalOrder: childrenInTraversalOrder,

engine/src/flutter/lib/ui/semantics.dart

@@ -818,8 +818,8 @@ abstract class SemanticsUpdateBuilder {
     required List<StringAttribute> decreasedValueAttributes,
     required String hint,
     required List<StringAttribute> hintAttributes,
-    String? tooltip,
-    TextDirection? textDirection,
+    required String tooltip,
+    required TextDirection? textDirection,
     required Float64List transform,
     required Int32List childrenInTraversalOrder,
     required Int32List childrenInHitTestOrder,
@@ -888,8 +888,8 @@ base class _NativeSemanticsUpdateBuilder extends NativeFieldWrapperClass1 implem
     required List<StringAttribute> decreasedValueAttributes,
     required String hint,
     required List<StringAttribute> hintAttributes,
-    String? tooltip,
-    TextDirection? textDirection,
+    required String tooltip,
+    required TextDirection? textDirection,
     required Float64List transform,
     required Int32List childrenInTraversalOrder,
     required Int32List childrenInHitTestOrder,
@@ -927,7 +927,7 @@ base class _NativeSemanticsUpdateBuilder extends NativeFieldWrapperClass1 implem
       decreasedValueAttributes,
       hint,
       hintAttributes,
-      tooltip ?? '',
+      tooltip,
       textDirection != null ? textDirection.index + 1 : 0,
       transform,
       childrenInTraversalOrder,

engine/src/flutter/shell/platform/embedder/fixtures/main.dart

@@ -167,6 +167,7 @@ Future<void> a11y_main() async {
       decreasedValue: '',
       decreasedValueAttributes: <StringAttribute>[],
       tooltip: 'tooltip',
+      textDirection: TextDirection.ltr,
       additionalActions: Int32List(0),
     )
     ..updateNode(
@@ -199,6 +200,7 @@ Future<void> a11y_main() async {
       decreasedValue: '',
       decreasedValueAttributes: <StringAttribute>[],
       tooltip: 'tooltip',
+      textDirection: TextDirection.ltr,
       additionalActions: Int32List(0),
       childrenInHitTestOrder: Int32List(0),
       childrenInTraversalOrder: Int32List(0),
@@ -235,6 +237,7 @@ Future<void> a11y_main() async {
       decreasedValue: '',
       decreasedValueAttributes: <StringAttribute>[],
       tooltip: 'tooltip',
+      textDirection: TextDirection.ltr,
       additionalActions: Int32List(0),
     )
     ..updateNode(
@@ -268,6 +271,7 @@ Future<void> a11y_main() async {
       decreasedValue: '',
       decreasedValueAttributes: <StringAttribute>[],
       tooltip: 'tooltip',
+      textDirection: TextDirection.ltr,
       childrenInHitTestOrder: Int32List(0),
       childrenInTraversalOrder: Int32List(0),
     )
@@ -364,6 +368,7 @@ Future<void> a11y_string_attributes() async {
       decreasedValue: '41',
       decreasedValueAttributes: <StringAttribute>[],
       tooltip: 'tooltip',
+      textDirection: TextDirection.ltr,
       additionalActions: Int32List(0),
     );