Remove incorrect non-nullable assumption from `ShapeDecoration.lerp` (#129298)

Fixes https://github.com/flutter/flutter/issues/129299

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

@@ -237,7 +237,7 @@ class ShapeDecoration extends Decoration {
     return ShapeDecoration(
       color: Color.lerp(a?.color, b?.color, t),
       gradient: Gradient.lerp(a?.gradient, b?.gradient, t),
-      image: t < 0.5 ? a!.image : b!.image, // TODO(ianh): cross-fade the image
+      image: t < 0.5 ? a?.image : b?.image, // TODO(ianh): cross-fade the image
       shadows: BoxShadow.lerpList(a?.shadows, b?.shadows, t),
       shape: ShapeBorder.lerp(a?.shape, b?.shape, t)!,
     );

packages/flutter/test/painting/shape_decoration_test.dart

@@ -48,6 +48,14 @@ void main() {
     expect(identical(ShapeDecoration.lerp(shape, shape, 0.5), shape), true);
   });
 
+  test('ShapeDecoration.lerp null a,b', () {
+    const Decoration a = ShapeDecoration(shape: CircleBorder());
+    const Decoration b = ShapeDecoration(shape: RoundedRectangleBorder());
+    expect(Decoration.lerp(a, null, 0.0), a);
+    expect(Decoration.lerp(null, b, 0.0), b);
+    expect(Decoration.lerp(null, null, 0.0), null);
+  });
+
   test('ShapeDecoration.lerp and hit test', () {
     const Decoration a = ShapeDecoration(shape: CircleBorder());
     const Decoration b = ShapeDecoration(shape: RoundedRectangleBorder());