Fix TextSpan gc regression (#126382)

`for (final InlineSpan child in children ?? const <InlineSpan>[])` bad

Fixes https://github.com/flutter/flutter/issues/126337

packages/flutter/lib/src/painting/text_span.dart

@@ -289,13 +289,16 @@ class TextSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotati
         builder.addText('\uFFFD');
       }
     }
-    for (final InlineSpan child in children ?? const <InlineSpan>[]) {
+    final List<InlineSpan>? children = this.children;
+    if (children != null) {
+      for (final InlineSpan child in children) {
         child.build(
           builder,
           textScaleFactor: textScaleFactor,
           dimensions: dimensions,
         );
       }
+    }
     if (hasStyle) {
       builder.pop();
     }
@@ -311,21 +314,27 @@ class TextSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotati
     if (text != null && !visitor(this)) {
       return false;
     }
-    for (final InlineSpan child in children ?? const <InlineSpan>[]) {
+    final List<InlineSpan>? children = this.children;
+    if (children != null) {
+      for (final InlineSpan child in children) {
         if (!child.visitChildren(visitor)) {
           return false;
         }
       }
+    }
     return true;
   }
 
   @override
   bool visitDirectChildren(InlineSpanVisitor visitor) {
-    for (final InlineSpan child in children ?? const <InlineSpan>[]) {
+    final List<InlineSpan>? children = this.children;
+    if (children != null) {
+      for (final InlineSpan child in children) {
         if (!visitor(child)) {
           return false;
         }
       }
+    }
     return true;
   }
 
@@ -393,7 +402,9 @@ class TextSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotati
         recognizer: recognizer,
       ));
     }
-    for (final InlineSpan child in children ?? const <InlineSpan>[]) {
+    final List<InlineSpan>? children = this.children;
+    if (children != null) {
+      for (final InlineSpan child in children) {
         if (child is TextSpan) {
           child.computeSemanticsInformation(
             collector,
@@ -405,6 +416,7 @@ class TextSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotati
         }
       }
     }
+  }
 
   @override
   int? codeUnitAtVisitor(int index, Accumulator offset) {