update BorderTween nullable (#79919)

packages/flutter/lib/src/widgets/implicit_animations.dart

@@ -152,11 +152,7 @@ class BorderTween extends Tween<Border?> {
 
   /// Returns the value this variable has at the given animation clock value.
   @override
-  Border? lerp(double t) {
-    if (begin == null || end == null)
-      return t < 0.5 ? begin : end;
-    return Border.lerp(begin!, end!, t);
-  }
+  Border? lerp(double t) => Border.lerp(begin, end, t);
 }
 
 /// An interpolation between two [Matrix4]s.

packages/flutter/test/animation/tween_test.dart

@@ -139,6 +139,17 @@ void main() {
     expect(animation.toStringDetails(), hasOneLineDescription);
   });
 
+  test('BorderTween nullable test', () {
+    BorderTween tween = BorderTween();
+    expect(tween.lerp(0.0),  null);
+    expect(tween.lerp(1.0),  null);
+
+    tween = BorderTween(begin: null, end: const Border(top: BorderSide()));
+    expect(tween.lerp(0.0),  const Border());
+    expect(tween.lerp(0.5),  const Border(top: BorderSide(width: 0.5)));
+    expect(tween.lerp(1.0),  const Border(top: BorderSide()));
+  });
+
   test('SizeTween', () {
     final SizeTween tween = SizeTween(begin: Size.zero, end: const Size(20.0, 30.0));
     expect(tween.lerp(0.5), equals(const Size(10.0, 15.0)));