Fix AboutListTile having an empty icon placeholder when no icon set. (#43422)

packages/flutter/lib/src/material/about.dart

@@ -112,7 +112,7 @@ class AboutListTile extends StatelessWidget {
   /// values default to the empty string.
   const AboutListTile({
     Key key,
-    this.icon = const Icon(null),
+    this.icon,
     this.child,
     this.applicationName,
     this.applicationVersion,

packages/flutter/test/material/about_test.dart

@@ -449,6 +449,26 @@ void main() {
     expect(rootObserver.dialogCount, 0);
     expect(nestedObserver.dialogCount, 1);
   });
+
+  testWidgets("AboutListTile's child should not be offset when the icon is not specified.", (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: AboutListTile(
+            child: Text('About'),
+          ),
+        ),
+      ),
+    );
+
+    expect(
+      find.descendant(
+        of: find.byType(AboutListTile),
+        matching: find.byType(Icon),
+      ),
+      findsNothing,
+    );
+  });
 }
 
 class FakeLicenseEntry extends LicenseEntry {