Use null-aware elements in widgets/routes.dart (#181242)

## Description

Adopts null-aware element syntax (`?x`) in `widgets/routes.dart`,
replacing 2 instances of the verbose `if (x != null) x!` pattern in the
`_ModalScopeState` widget.

Towards #172188

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

[Contributor Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md

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Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

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

diff --git a/packages/flutter/lib/src/widgets/routes.dart b/packages/flutter/lib/src/widgets/routes.dart
index 0e00d2f..dc3545b 100644
--- a/packages/flutter/lib/src/widgets/routes.dart
+++ b/packages/flutter/lib/src/widgets/routes.dart
@@ -1098,10 +1098,7 @@
   @override
   void initState() {
     super.initState();
-    final animations = <Listenable>[
-      if (widget.route.animation != null) widget.route.animation!,
-      if (widget.route.secondaryAnimation != null) widget.route.secondaryAnimation!,
-    ];
+    final animations = <Listenable>[?widget.route.animation, ?widget.route.secondaryAnimation];
     _listenable = Listenable.merge(animations);
   }