Fix prefer_null_aware_operators violation (#132242)

Use a null-aware `?.` operator instead of a conditional operator. This complies with the `prefer_null_aware_operators` rule. Fixes https://github.com/flutter/flutter/issues/132241

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

@@ -707,7 +707,7 @@ class RadialGradient extends Gradient {
       radius * rect.shortestSide,
       colors, _impliedStops(), tileMode,
       _resolveTransform(rect, textDirection),
-      focal == null  ? null : focal!.resolve(textDirection).withinRect(rect),
+      focal?.resolve(textDirection).withinRect(rect),
       focalRadius * rect.shortestSide,
     );
   }